By 8wks, most of my usual bras didn't fit anymore, and were all underwire. I went to bras and things, and they said I could either get fitted for a mat bra then, or go by wireless tshirt / sports bras from kmart etc... I went the mat bra option, and have been loads more comfortable ever since. I have heard the time to be fitted is around 20wks, so some people are warning me I may grow out of the ones I've bought. Oh well, will cross that bridge when I get to it. Atleast, I'm heaps more comfy, and that is what's most important.
Hope that helps!