Measuring from the Admiralty 1/48 builder's drawings the wheels are 122mm in diameter and the rims are spaced 28mm apart. They are nine-float wheels and the floats measure 57mm x 21mm. The sizes may vary slightly from drawing to drawing as a result of errors in copying and the possible instability of the originals.
I don't believe that there are any wheels commercially available. I did some sketches for my wife who then did CAD drawings for the wheels, eccentrics and arms, float mounts, rivet detail and other detailing parts for the rest of the vessel and we had them photo-etched by a manufacturer who is happy to do one-off's for model-makers.