Alpine Type S, although rather mainstream, do offer some nice sounding speakers and can be found pretty cheap. My brother and my buddy have P1 Rockford components in their GP's and they sound alright. I'm also a fan of of Boston Acoustics and Diamond Audio but their entry level stuff is a little more retail but does have better value over Alpine or Rockford. Check Ebay.
EDIT: checked ebay and they seem to have mostly 6.5" components which can be modded a bit to fit but offer better mid range.
http://cgi.ebay.com/NEW-DIAMOND-AUDI...d=p3286.c0.m14
http://cgi.ebay.com/DIAMOND-D362-5-C...d=p3286.c0.m14
http://cgi.ebay.com/DIAMOND-AUDIO-M3...d=p3286.c0.m14
http://cgi.ebay.com/Diamond-Audio-D6...d=p3286.c0.m14
http://cgi.ebay.com/DIAMOND-AUDIO-D3...d=p3286.c0.m14
