The product you link has the cap in the picture.
The idea is that you cut off the tip on the apex of the cap to the point where it is the width of your desired opening.
You could use an uncut cap, or just cover the tip with foil if you need to refrigerate for some time.
The kind of things put into a squeeze bottles are rarely highly perishable, so simply placing them in refrigerator (for a reasonable period of time) is a normal practice.
Squeeze bottles are not meant for long term storage.
I certainly would not go to the trouble of special ordering caps.
cooking.stackexchange.com/users/14401/SAJ14SAJ
I've been searching for a while.
I use my squeeze bottles for various things, and the tip caps are helpful.
It keeps royal icing from drying out, and prevents leaks when shaking up a bottle of vinaigrette.
Here's what I found: http://www.usplastic.com/catalog/item.aspx?itemid=91498&catid=536 http://www.dharmatrading.com/tools/squeeze-bottles-yorker-spouts-and-caps.html
cooking.stackexchange.com/users/23346/OtterQueen
Similar bottles will sometimes come with a little red or black plastic cap that covers the tip of the bottle after it has been cut.
If the bottles you bought didn't come with these caps, I don't know that there's a way to get them separately.
Occasionally a restaurant supply store will let you switch out bottle tops when buying the bottles, but other than that, I think you may be out of luck.
Frequently to close up the bottles overnight or between uses, you take off the screw top with the spout, place plastic wrap over the top of the bottle, and then screw the top back on.
As SAJ14SAJ points out, bottles like this aren't really intended for long term storage and caps generally aren't a priority.
Edited to add: The restaurant supply store nearest my house now sells the caps separately.
They are in a package of about a dozen, but very cheap.
cooking.stackexchange.com/users/16863/SourDoh
You might find something useful at US Plastics, for example at: http://www.usplastic.com/catalog/item.aspx?itemid=91498&catid=536
These are the whole twist-on cap, not just the snap-on tip but they are inexpensive.
cooking.stackexchange.com/users/20552/PhilipD
Crayola's Model Magic is non-toxic (with a lot of sites claiming it's food safe).
If you have access to that or similar clay, you could make your own cover, no matter the size.
(but I don't suggest raiding your kids' supply ... get a fresh container, use what you need, then let the kids get their hands in there)
cooking.stackexchange.com/users/67/Joe
Here's what I ended up using, which I picked up from Home Depot in the Hardware aisle.
It fits quite snug, and cost about $0.60 for pack of 2. WARNING: no guarantees this cap is safe for food.
Use at your own risk (even though it is molded plastic, the dyes and treatment process may make the plastic prone to leaking chemicals)
cooking.stackexchange.com/users/7219/Dolan Antenucci
I purchased some  Bakers and Chefs clear plastic squeeze bottles , which do not come with caps; however, there is a ridge about 1/3" down on the bottle, suggesting that they're intended to work with a cap.  Some of the reviewers on Amazon mentioned using foil, as well as some plastic cap they found in hardware store. I'm hoping to find something that fits well and ideally can be ordered online.    Anyone have any suggestions?
Any suggestions on caps for Bakers and Chefs plastic squeeze bottles?
cooking.stackexchange.com
<equipment>
cooking.stackexchange.com/questions/37282
cooking.stackexchange.com/users/7219/Dolan Antenucci
While you may be able to find a supplier that will sell you caps or allow you to switch out bottle tops when buying the bottles, these are not intended for long-term storage and are normally just covered with plastic wrap overnight or between uses. Squeeze bottles sometimes come with caps. If they do not, you can get caps at a restaurant supply store. There are, however, other ways of closing the bottles, using plastic wrap for instance.
Some bottles come with the caps. You can make a cover using plastic wrap. The bottles are not intended for long term storage so caps are not a priority. Some restaurant supply stores allow you to change caps or they sell the caps separately.