I bought my daughters training bra at Kohl's. They are in the little girls' panties section. What you are trying to do is basically, for lack of better words, smooth them over under their blouse. Don't look for anything fancy, something similar to a cami (in a bra size) is good. Here is a link to the exact kind I purchased. They worked great. Another thing to keep in mind is this is the time in their lives that boys notice bras and poke fun. Make it as less obvious as possible.
My daughter, 9 years old, also started to develop buds this year. The doctor recommended a sports bra for girls with a padding to protect them, particularly from getting hit with a ball in gym class. I found several at target in the girls section. They sell them in girls sizes like 7/8 & 9/10. They are very cute & she loves them.
My 10 year old has nothing there yet but is very active and feels more comfortable when she wears a sports bra. We found hers at Kohls in the girls section, like Christine mentioned. They have a pretty good selection.