Ouch I remember how you feel (even now, although the ache has gone the size is still annoying!).
I got maternity bras at around 6 weeks. I don't wear underwired anyway, but even my normal ones were getting tight, and night time I found was the worst. I got two sleep bras back then and am still comfortable in them now (too comfortable, I spend half the day in them as well sometimes!). Plus a couple of soft but supportive ones and a slightly more padded one for going out.
However, getting 5 bras from mothercare was a costly experience, so I started looking in tescos and sainsburys and got much cheaper sports bras that did almost the same job. Plus I got some bra extenders (couple of quid from john lewis) and managed to use my own original ones and still do around the house sometimes.
In regards to when the pain eases off, probably somewhere into your second trimester you'll notice that suddenly walking down the stairs doesn't hurt so much. I know it just eased off without me realising until one day I bounced down the stairs and realised it didn't hurt!
I found that mine grew pretty quick and I don't think my BBs themselves have grown much more but my chest possibly has. Either that or the cup size was quite generous to start with. Although I'm sure they'll increase a bit more soon. I read to wait until about 36 weeks before going for nursing bras so that your size is about the right size for them.
Hope the pain eases soon!