I don't think that the gear you referenced is for a 1/8 inch shaft. A 10 tooth 48 pitch pinion is about as small as you can get on a 1/8 inch shaft and your source has pinions down to 7 tooth on the same ad and is for slot cars.
I bought some 10 tooth pinions directly from Leisure Electronics. I forget the price, but it wasn't much. I don't have their address at hand but I remember I found it easily by doing an internet search.