We have two sources of bases.
1) Contact Mark (Sacramento, CA) by phone at (916) 285-5593 or via his website.
2) Contact Roy at (818) 705-4693 from Marble & Bronze (Resida, CA).
Woodcraft the woodworking tool supply chain stores sell 2"x9"x12 dark grey granite block. They sell it as sharpening surface plate, a lapping stone and etc. Fairly cheap.
5) BlackMarble@pacbell.net San Marcos CA