I got a pair that fitted well in H-cup from thebreastformstore.com, their largest at the time, called Supergirls. It was tough finding quality big forms back then. It looks like they've increased their range. Cost is at the top end but I can attest to their quality. They opened a UK store to cut shipping costs. They may have more outlets now - their site says they ship to US/Canada/Americas, Continental Europe, Germany, UK, Australia, New Zealand and Japan.
I bought a pair from Forever Young on Amazon, which were cheaper, but I found their sizes weren't as big as stated. They said J-cup, which were actually G. It looks like there are more cheaper suppliers on Amazon now but I haven't checked sizes.
I wore mine sideways, so the top of the triangle was at my underarm, which gave them a more fake look.