Try some of the BalTogs custom stuff. You can order through www.nydancewear.com. Get their paper catalogue. There are a number of tank top styles, and you can pay extra for a thong butt-strap it it's not part of the original style.
I have two BalTogs short sleeved leotards with a thong butt-strap. The backs are definitely high. I don't have a BalTogs tank-style, but one of the many styles they have must have the back you're looking for.
Good luck! Keep in touch!
balletboy[/i]