I don't know if this helps, and I will try to explain as best as I can... because I don't know what they are called (I tried googling and came up with bubkus:o)
When I was preggers, I was a funny size. Because I was sooooo tiny (103lbs before preggers) and have such delicate bone structure, most mat clothes fit me funny. They fit in the belly but nowhere else... which really didnt work.
What I did, often, was buy summer dresses at a department store, or blouses that were a size bigger. Then use a special clip to cinch it in the back, above my waist and below my bust line... so that it fit in the bust area and was loose in the belly.
They are usually about 2-3 inches long, with 'clamp' like clips on either end and 1/5inch wide. Sometimes made of fabric with elastic inside. Sometimes with just beading or other ornamentation. The clips resembled those on suspenders, quite a bit.
WHOOPS FOUND ONE:
Vintage Clothing Scarf Blouse Jewelry Clip - (eBay.ca item 320248737043 end time 08-May-08 15:19:09 EDT)
You can also make fabric strips that you can attach to the back so that you can tie it in a bow to 'cinch' it and pull it tighter or looser as necessary? (not sure if I'm making any sense)
Maternity clothes where I am are simply TERRIBLE and mucho expensive. Not much choice because most companies can't be bothered with making bilingual labels "just" for Quebec. MOst mother's here, leave the province and go shopping even in the US for mat clothes.
This might help too, an idea for altering your own stuff:
Mafe Maria • DIY Pregnancy Jeans
hth's