JayceeTG
I would caution you that clothing you find on TG specific websites is often of inferior quality and overpriced. You would certainly be better served by purchasing cloths from a not fetish local retailer, such as Khol's, Target, Walmart, or even Goodwill if you are starting out.
Most retailers do not have issues with your trying on whatever clothing you like. Going during the morning is a good time as there are fewer customers, and certainly less people inclined to make snide remarks.
As for decent breast forms, your best bet if you can afford it, is to go with a prosthetic breast form manufacturer such as Amoena or Nearly me. Lesser forms can be found, tend to be of significantly lesser quality.