You can find them in most sporting goods/athletic wear stores, but I've only found one or two styles of sneakers in any store and I was not looking for sneakers.
I just bought a pair of Merrell boots from shoes.com and a pair of mary janes from zappos (shoes.com was cheaper, but didn't have my size). They both offer free shipping and free returns, so if you could narrow it down to 2 or 3 that you think you will like you can try them and send the other(s) back. I usually wear an 8.5 and the Merrells in that size fit me well, so you shouldn't have to worry too much about sizing.