One of the easiest ways to locate used class C Winnebagos for sale is to find a reputable Winnebago dealer. Many of them sell both new and used RVs.
Class C motor homes are also often for sale on websites such as RVTrader.com or auction sites such as eBay.com. These sites let you filter search results by manufacturer and size as well as other criteria including location. It's usually safer to buy used vehicles locally, which allows you to test drive them and have them inspected by a mechanic you trust.
You can look at local newspaper classified advertisements or on classified sites such as Craigslist for listings. These sources often group all the RVs for sale together, so it may take more effort to search for class C Winnebagos in particular. However, they can be a good place to find deals.
If you do buy an RV from a private seller, be sure to do plenty of research before finalizing the sale. Make sure the engine runs and the living space is in good condition. Look for tears, fading, punctures and other damage that may cause leaks. Make sure all the lights and electronics work. It's also a good idea to make sure the RV is priced at a fair value. This can be done by looking at the selling prices of similar RVs and checking an RV valuation website such as NADAguides.com.