if you want fake plants, silk plants are good because they're soft and won't harm bettas' fins. if you want to go with plastic plants, just make sure that they're soft and not rough and won't snag onto bettas' fins.
you can test this with pantyhose, just rub it on it and if it snags it's too rough.
i bought these plastic plants and they're great:
i tried to find the petsmart link but i couldn't find it.
they're the penn-plax multi-pack, i bought one of those, and another penn-plax mult-pack with different colored plants.
OH, the only thing i don't like about those plants is that they don't stay down, they FLOAT.
so i had to put some small rocks in their bases for them to stay down. they look pretty and they're soft, but if you don't want to go through the trouble of trying to make them stay down, don't buy those.
it's also good to get them some kind of cave, so they can hide or rest during the day. make sure anything that has a hole, you can fit your thumb through it so your betta won't get stuck.
i have this cave for my betta:
he loves it.
if you want to get a substrate (gravel, sand, stones etc.) makes sure that they're also smooth, and they don't contain any toxic metals or chemicals. i recommend some flat-bottomed marbles. if you choose to go with a bare-bottom tank, that's fine since it makes it easier to clean.
if you have any other questions you can read through this thread: