"I was wondering if anyone had any suggestions as far as any vendors are concerned that custom make these tops. "
Hey, you asked! I make custom halters, up to a 46DDD so far. tribes.tribe.net/angeldmort
tribes.tribe.net/angeldmor...7bfad7bff6

The tops themselves aren't hard to make, themselves, once you have the pattern figured out, but anything larger than a B is going to need darts and curves and support, so the whole 'simple triangle of fabric' idea starts to fail. Most problems come from using too narrow a triangle which ends up not covering at the sides once you start pinching it in under the boobs and then on the sides to avoid the 'side fallout.' Next is the 'pointy boob' problem, where the darts bring it in underneath, but make a noticeable point. And the 'flat boob' where the need to contain things results in making the top tight instead of fitted.
Do you want to make one, or just have one? If you aren't wanting to say 'I made this' then yes, drop me a line, and we'll get you set up with a triangle halter. For that matter, drop me a line if you want to make it, too. I'll do what I can to help via email.