I'm in search of this myself. What I have done was contact Dangerboy(the company that makes our fitness wear for the videos)and ask him to make me a couple of maternity tops. He said that he has never done this before so I'll let you know how they turn out.
Japanese Weekend is a maternity clothes catalog and they make a bike short with really good support for your expanding belly. You can order online from them at www.japaneseweekend.com look at their online catalog under activewear. Good luck!
I just got my "demo" maternity top in the mail. It fit almost perfectly. Just a little snug in the belly area. The top was so cute that I would never have guessed that this was his very first attempt at making maternity fitness wear. I called him and told him plus gave him my new tummy measurement on the phone. He is sending another demo out in the mail tomorrow. Aside from the demo being snug, the material was very comfortable and supportive. It was made to add your own sports bra underneath. You can also select whether you want a racer back style or a standard back(like a bra cut). I asked him if he would like to add maternity fitness wear to his fashion line and he said most definitely. If you would like to call Dangerboy, their number is 619-299-8204. Have fun!