I'm a size 28F. No joke.
Super skinny parents, just I got the big breast genes from my mom's mother.
I go to figleaves.com which has many bras in a wide range of sizes.
Unfortunately, many bras that I buy are fairly expensive, but it's worth it to be comfortable and have a bra that fits and supports.
Good luck!
(I'd be interested to know about custom bras too, actually.)