All Bettas need a plant in their tank to make them feel safe. I wouldn't go for a living plant in a 1 gallon tank, but a nice silk plant would work. Make sure it's soft, made of silk, not plastic. The hard, plastic plants can tear their fins.
Though in a one gallon... you can't really fit more than 1 plant. Maybe two smaller ones.
The only place I've been able to find silk plants here is PetCo, and they've got a rather small selection. Wal-Mart sometimes carries some, but they aren't good quality and they break in no time.