Most sizable cities have dealers that sell breast forms. Look in your yellow pages (or the online equiv.) for "breast forms". Call and ask if they work with TG people. There are stores that deal primarily in things for crossdressing and TG people that sell them also. There aren't many around, though, so you may have to travel to visit one. You don't say where you are, but I know in Germany there is shop that sells forms exclusively and has a huge selection you can pick from in their showroom. I'm sorry, but the name slips me.
Most people just take their chances with shopping online when it comes to these things. You want to take careful measurements and try to buy a well recommended product, especially if it's one of the more expensive ones. There are some lower priced forms sold on eBay and Amazon some people here have used and found acceptable. It might be preferable to go that way initially just to test things out for yourself. My first forms were the inexpensive ($20) "cutlet" type sold to give women an extra inch or two. They're smallish, but many ladies are.