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Bought mine from howard. He is a member here, though he is not much online these days. His service is excellent, and his prices are ok.

I really advise you to buy the DMT8C.
It only costs 55$, and you can use it to lap any stone out there.
Sandpaper would work if you occasionally hone a razor. But as soon as you start buying and...

I use a DMT8C (325 grit) to lap all my stones, and it works wonders.
Don't worry about the grit size too much. The 'coarse' scratches don't matter, because the material still scratches your razor...