I feel your pain... I am a 30I at the moment & NOT preggo or nursing!!! I have done a lot or research & tried a few things but gave up. Even in RTW bras I don't find enough support in the plain fabric or lace kind! I prefer a thin molded (not padded) seamless t-shirt bra & I can't sew those!
I used to cry about the $ of the bras, but honestly I buy 1-2 a year on sale that are good comfy supportive bras & decided I was worth it. I have found that I dislike the ones I can find in my size in the $50 price range. Most of mine are sadly $60-$75 though I get them on sale for $40-$50 sometimes. It's worth the investment for me because between the cost of supplies & the time it would take it's just not worth it for me personally.
This is an AWESOME US site that carries lots of Euro & UK bras. They also allow returns

. I have gotten my favorite bras on sale for a decent price in the past & they often have free shipping or other specials. I think they carry up to a 36N? The don't have many options in that size right now but they get more at times.
http://www.barenecessities.com/
I understand with your size being even harder to find wanting to try to sew them. The hardest thing will be finding a pattern or book that goes up that high in sizes... I had a hard time finding anything that went over J. If you do have a bra to spare taking it apart may be the best option.
Good luck!