How much are you looking to spend?
I bought my first viv at a reptile shop when I purchased my baby female, and got a great deal on it. Lately I've been looking for a new one for my male, and to get one at a cheap price I've been looking on craigslist.com. It's a hit or miss, just have to get lucky. Maybe check out local pet shops for postings of viv's for sale on the bulletin board.
Otherwise, I can only think to find them in a pet shop or reptile store. You'll be paying a bit more that way though. :?