If you can get some, water lettuce would be great :D They grow large and have long hanging roots that provide cover at the top of the tank, which is needed since bettas are top level fish.
Silk plants can be bought inexpensively at most craft stores, and make great suppliments as well. However, there can be no exposed metal, and you'll want to soak them in a bin of water overnight to make sure they don't leak color into your tank. If they leak any color at all, then you'll want to try a different silk plant.