Frank, the biggest thing you need to do is find one that is not worn. Using a dial indicator on the stem of a valve, measure the concentricity of the arbor. Then rotate the valve 90° in chuck and measure again and see what the run-out is. A good machine within tolerance will likely be in the $1,000.00+ range.
As for buying replacement stones,
Goodson is who I use. Not cheap but it should last you for awhile.