Consider getting some live plants (I mentioned this in your other thread). They'll help with ammonia and aren't as difficult to take care of as one might think. Even some java moss or a marimo moss ball are super-duper easy.
If you'd really prefer to stick with fake plants, just make sure they are soft so as not to tear delicate betta fins. Silk plants are preferred over plastic. If in doubt, scrunch it with your hand - if it hurts/digs in, then it is more likely to hurt betta fins.
Here's some examples of silk plants:
I also tend to think they look more natural than the plastic kind, but that's just me :>