Virtually any of the smaller shrimp (Ghost, Cherry, Japonica, etc) but I'd be more cautious with something the size of a Bamboo Shrimp. Is there going to be heating?
Defiantly no bamboo shrimp. Bamboo shrimp filter feed to need a relatively large and mature tank with a filter. I think your best bet is cherry or tiger shrimp. It really depends on the water in the tank, but for a 2gal you need something small, i think amanos are too big and i think even the 3.5 gal i had two in was slightly too small for them.
Choose from any of the small shrimp here as long as your water suits them.
I've been told by a lot of betta keepers that bettas usually kill and eat shrimp. I don't know if I would try it with anything that would make you upset if it became a meal.