RCU does have a category for retracts; it's a sub category under airplanes-
Airplanes > Accessories > Landing Gear & Tires
http://www.rcuniverse.com/market/category.cfm?catID=298
If you need help submitting an ad, we have tutorials for you, available by clicking the HELP tab in the marketplace-
http://www.rcuniverse.com/market/help.cfm
Your other option is to search "robart" or other search terms in the marketplace to find what is currently for sale.