I don't think there is any realistic prospect of finding a New Old Stock pulley anywhere. The chances of finding a scrap Supercut with a good pulley are not quite zero but pretty close to it, and you have taken the right step by posting your need for it here, though the "Wanted" forum might have been a better place.
If all else fails you can either repair your old pulley, or have one made. It should be a routine lathe process except for the grubscrew tapped hole and the internal keyway, assuming it has those features. However before resorting to having one made, you should see whether a supplier of industrial V belt pulleys happens to have one to suit. This could easily be the case if SB chose to use a standard industrial product on the mower, rather than re-invent the wheel by designing a special one. Here is an example of the type of supplier I have in mind:
http://au.rs-online.com/web/c/pneum...82a&gclid=CPrF_qqYgcYCFdd5vQodRgQA6wI do not know or recommend that supplier, they are merely an example of the type of seller you are looking for. Note there are also some thousands of different V belt pulley types on offer on ebay.
