Cream does last quite a while in the fridge, especially before it's opened - it has a pretty high fat content.
So you may be able to buy it when you can, and still have it around when you want it.
Failing that, honestly, I think that anything you can do with milk and additives is probably not going to be nearly as good as actual whipped cream, so you might as well fall back to another substitute: aerosol whipped cream.
If you get a decent one that's actually made with cream (not milk and oil and stabilizers, like your Dream Whip)
it'll probably still taste better.
And I'm pretty sure it lasts a really long time (months?)
in the fridge even once opened.
(But I don't have any to check for sure.)
As for actually making Dream Whip: as mentioned in several comments, it has a lot of ingredients that'll be way harder to find than cream.
Sure, they're probably shelf-stable and you could probably buy them online, but it'll be a lot of trouble, and it might well be more expensive than just buying the Dream Whip itself.
It's possible that there's a more home-ingredients version (I'm no industrial food authority), but unless you have some more restrictions I don't know about, I'd still prefer the other alternatives I suggested.
cooking.stackexchange.com/users/1672/Cascabel
This site explains two methods of making whipping cream from milk.
The first is by manipulating store-bought milk, by adding gelatin, temperature and a lot of stirring.
The second is by separating the cream out of raw milk.
The site claims that the result of both methods can be used for whipped cream.
A third method I sometimes use when a recipe asks for a small amount of cream (usually in a sauce or something similar), is adding some milk and butter (till the fat percentage is roughly the same as if you'd added cream).
I don't believe it would be possible to whip this though.
cooking.stackexchange.com/users/4580/Mien
Alternatively you can use coconut milk.
Use canned coconut milk-make sure it straight milk and not sweetened-and place it in the fridge until well chilled.
Scoop off the firm layer of cream, making sure not to include the water which should be at the bottom of the can.
Add in vanilla and sugar if desired, whip until soft peaks.
cooking.stackexchange.com/users/19397/ganache
Per Food facts : Sugar, Dextrose, Vegetable(s) Oil Partially Hydrogenated (Coconut Oil Partially Hydrogenated and, Palm Kernel Oil Partially Hydrogenated) , Corn Starch Modified, Propylene Glycol Monostearate
An Emulsifier, Sodium Caseinate Solids from milk, Monoglycerides Acetylated An Emulsifier, Sodium Silico Aluminate (Anti-Caking Agents) , Mono and Diglycerides
An Emulsifier, Cellulose Gel, Cellulose Gum, Methylcellulose Hydroxypropyl, Flavoring Artificial and Natural, Yellow 5, Yellow 6
Some of these ingredients are not the type of thing that you are going to replicate at home.
cooking.stackexchange.com/users/14401/SAJ14SAJ
There are some vegetable based vegeterinan whipped cream substitutes.
But like with margarine vs butter or soy vs meat the tastes aren't exactly the same and most people prefer actual cream/butter/meat.
However whipping cream is a relatively standard product that any store should carry while the other is a niche product so if your store has no whipping cream it might not have that either.
Another obvious choice is whipped cream in a can.
While it's not as stable as cream whipped at home at least you might be able to enjoy some N2O.
cooking.stackexchange.com/users/54261/Mr. C
One alternate possibility to either dream whip powder or to fresh cream from the market is, well, there is heavy cream powder available for purchase - pretty much just dehydrated heavy cream - which can be re-hydrated and used for pretty much anything cream can be used for, including whipping.
The heavy cream powder is available for purchase online, I got mine from amazon.com but there may be other suppliers to be found if amazon does not suit.
And I have used it for whipped cream, and other recipes asking for heavy cream without issue.
Ordering online is not always as convenient as buying from a local store, but it is useful for things not readily available - especially shelf-stable things, which can be stored for quite some time and used on demand.
Unlike the fresh cream, heavy cream powder is quite stable and can be kept for a long time - as many dehydrated foods are - so purchasing one package would mean cream was available on demand until the powder was used up.
And unlike dream whip powder, it is not heavily modified or heavy with chemicals, and should therefore have a more authentic cream taste.
cooking.stackexchange.com/users/47365/Megha
I sometimes want to make whipped cream for dessert, but can't get cream in the market. What can I do in that case?  One possibility: I did find that there is a quick way to do it by mixing a commercial powder called  Dream Whip  and milk. What are the actual ingredients of  Dream Whip ? I suspect that it mainly contains some foaming agents like corn starch or gelatin, but I couldn't find a recipe as an alternative to commercial  Dream Whip  powder. Can I make that at home?
How can I make whipped cream if I can't always get cream?
cooking.stackexchange.com
<milk><cream><whipped-cream>
cooking.stackexchange.com/questions/35208
cooking.stackexchange.com/users/9300/Googlebot
You can manipulate milk by adding gelatin, heating and stirring, or separate the cream from raw milk. Coconut milk and vegetarian substitutes can also be used, or you could try squirty cream or powdered whipped cream, which is quite stable and has a long shelf life. Many substitutes for fresh cream are available on the market. One good option is cream powder that can be bought online and lasts for a long time.
There are many alternatives on the market to fresh cream, including vegetable-based cream substitutes, canned cream and powdered cream. Cream powder can be bought online and keeps for a long time.