Melody Leotards. Anyone can customize on their site. I have to stay away because it's addictive. You choose the style and add your colors, choose your back. You can contact them if you have a style you want that they don't show in the store. Their prices range from $20 to $120 for comp/long sleeve. She has quick turnaround and great customer service.
I think Ozone also customizes, but I don't know their costs.