This subject interests me a lot.. so i did some goggling and found the below.
Well, I suppose it goes without saying that you only have one 'right' size, and if that's 32G then a 32G bra is going to fit better than any other size. But I take your point, they're hard to find and expensive when you do, so if you're happy to compromise fit a bit in exchange for choice and value, then as a general rule you would need to increase the band size by one for every cup size you come down. So you would almost certainly be okay with a 34F, probably with a 36DD/E, maybe with a 38D... the further you take this, the bigger the compromise.
People will tell you it's important to only wear exactly the right size, but that's easy to say when you're an easy size and have endless reserves of cash - ultimately bras are not 'necessary' at all, they're a choice, and if you want to experiment with sizes, go for it.
If you need help figuring it all out like Drozek said post a pix and I can suggest other sizes as well