Buy from a store that sells lawn mowers and snow blowers only, because they will have the parts to repair/service them when they break down. The most important thing to remember is to use all the gas up before storing it, when your done using it for ...
Try eBay. If it's not listed, email one of the sellers to see if they have the part.
check your local classifieds; check with a dealer who takes trade-ins. Check with an on-line site such as Craigslist or the auction sites for completed sales.