Some Petsmarts are selling Marimo Moss Balls (which is actually an algae, not a moss) in little cups right next to the bettas. Those are super simple to keep and run about $8.
Petsmart has quite a few softer plastic plants (like the 8" corkscrew) that run about $3 each. Feel each plant and make sure it is soft and has no sharp edges. You may want to trim them with scissors to make them suitable.
Petco has a package of silk betta plants, something like 3 silk plants together for about $6.
I'm a fan of the plastic amazon sword plant that Wal-Mart sells for about $4. You may need to trim the tips carefully to make sure there are no sharp points or edges.