While McMaster does have G-10, I have used their "regular" GRP board for things like backing blocks. It's laid up with polyester and not epoxy, but since it's all a secondary bond and since there is not extreme strength required, I haven't seen the need for G-10 for something like a backing block (although I would use it if I felt it was necessary or better). Depending on where you live, McMaster orders usually arrive the next day or perhaps the day after (regular shipping).

http://www.mcmaster.com/#fiberglass-%28frp%29/=hl63uy

It's the basic, opaque green (vs. the G-10 which is sort of a light yellowish green, or the electrical grade which is red).