FR-4 is used almost exclusively, and is very inexpensive.
Check with any of the board fab houses. A couple
advertise on RF Cafe (no, I'm not one of them).
From a quick look here are the ones I see (might
as well support them since they essentially pay
for this website). Sorry if I missed a couple. I
think they all specialize in quick turn-around and
even small quantities.
in beautiful Wisconsin