Betta Tank Help...Please!?

I was wanting a betta or 2. I have a 10 gallon tank but... it already has three fish in it. Two serpae tetras and 1 neon tetra. I also have a vase that i had an old betta in. Any ideas what to put it in? Also any good fish stores or anything? I was going to go to Toms Tropicals! Thank you (I live in the Parkville,Maryland area)