Over the years I have purchased quite a few from Amazon. There is a wide variety of styles and sizes to choose from. I found that the silicone type makes me sweat underneath, and the bigger you go the heavier they are. So a good supportive bra is important.
I also found that many breast forms are designed for people with little or nothing on top. They are almost flat on the backside. Once I started developing, the forms no longer fit comfortably. Trying to find forms with a concave back is more difficult and as Heidimarie pointed out, more pricey. Now, if I want a little extra boost, I use the "Chicken Fillets" forms at the bottom of the cups to push the girls up. Sweating is still an issue (they are silicone) but is minimized because they do not cover the entire breast.