As you play the game, you will receive plants as random drops from zombies.
The store ONLY stocks marigolds and when you are collecting gold from your plants, the marigold drops gold far less often than plants earned from random drops.
That said, make sure you buy your three marigolds each day to fill up your garden.
Important:
Make sure you have space for at least one plant left in your garden.
If your garden is completely full, you won't be able to add plants that drop.
Delete marigolds BEFORE deleting special plants received from drops since dropped plants pay out gold at a faster rate than marigolds.
Marigolds just make a nice filler until you get enough drops to make the big dough.
gaming.stackexchange.com/users/6697/Melanie
Plants are rare drops for playing the puzzle, survival, and adventure modes after you beat the game.
Furthermore, the three sold in the shop reappear each day, allowing you to buy three per day.
gaming.stackexchange.com/users/3107/Shaun
You get extra plants while playing the main game after the Zen Garden has been unlocked.
You probably picked up the 2 other plants while playing.
They pop out kind of like coins, but much less often.
gaming.stackexchange.com/users/6066/Matthew Read
Play more survival, adventure and puzzle games to get more plants.
Survival gives more plants than adventure or minigames so it's best play survival.
gaming.stackexchange.com/users/17544/troi
If you get the latest version you will be able to buy 7 plants in the shop.
There will be 3 Marigolds, 1 plant that has a chance of getting the same plant, 25% chance of getting a new plant, 50% chance of getting a new plant, 100% chance of getting something you already have.
gaming.stackexchange.com/users/29586/somebody
Just randomly play minigames, I got most plants from last stand, one time I got 2 plants in 1 game.
gaming.stackexchange.com/users/39285/Neva Sufyani
you just have to be lucky you need to unlock zen garden and 99% of the time you get it by playing survival NOTE you need to beat the game first
i don't mean beat everything
i just mean beat DR.zomboss.
gaming.stackexchange.com/users/38156/hollowdreams9
I have recently unlocked the Zen Garden, but there are only 3 plants for sale in the shop and there is tons of empty room in the garden.  How do I fill up the garden?  Also, I have 2 grave-eating plants in my garden that just kind of appeared with no action taken on my part.  Why did this happen?
How do I get more plants for my Zen Garden?
gaming.stackexchange.com
<plants-vs-zombies>
gaming.stackexchange.com/questions/14737
gaming.stackexchange.com/users/1719/Invader Skoodge
Play more survival, adventure and puzzle games. If you get the latest version, you will be able to buy 7 plants. Playing the different game modes, like puzzle, survival and adventure after the Zen Garden is unloceked will randomly give you more plants. Of course you can always buy them in the shop too.
Users suggest that just playing different game modes will give random chances of receiving more plants.