I thought I'd post on how I went finding some enormous bras. I did eventually buy two, one of which wasn't entirely right and one that is pretty good.
The gist of this is that I'm breastfeeding and have breasts that are disproportionately large for my frame - I'm an Australian size 10 or a US size 32, and need a G cup or thereabouts. Cup sizes this big are very, very hard to find in band sizes this small. Since I had trouble finding what I wanted, this page is here to help anyone else who has the same problem I do.
Since bra sizes vary so much around the world, the first bra I bought was from an Australian site, http://www.brastogo.com.au/ . The maintainer of this site is great, and will ring you to discuss any issues you have. I bought a Royce Jasmine Maternity Bra (style 469) which comes in cup sizes F-L. It kind of sort of fit, but because it is made out of quite stiff fabric and has a ribbon and some lace for decoration, the ribbon actually cut into me and left a line. The style also has this really bizarre thing where it makes your breasts point away from each other, which is not particularly flattering (mind you, when you are wearing a G cup nothing looks very good) and feels very strange because they point so far away from each other that you hit your arms on your breasts as you swing them (your arms, not your breasts). So anyway, I kept looking.
After asking some advice on some forums, I wound up at http://www.bravissimo.com/ and bought myself a Rosalie Nursing Bra by Freya (style FYE2). This one comes in cup sizes from D-H and is entirely soft cup, with a bit of lace. It'll probably stretch out of shape in a few weeks of constant wear, but I can live with that. Its nowhere near as stiff as the Royce and it doesn't hoik you out sideways, it points you in the normal forwards direction. Both have identical full-cup releases for breastfeeding, and both come in more colours than just beige. So, imagine my wonderment at finally aquiring a bra in *gasp* black that fits and doesn't leave me with red lines or squashed nipples. I'm in shock, really I am. There are so very, very few bras out there in this size range.
Both images were taken from their respective linked sites.