Where can I find a free/cheap small airplane wing?
Wings may be tough to come by as they generally last the life of the plane and can be re-skinned/fixed if need be so they are not scrapped all that frequently. You can try a used parts dealer like this one or this one but they generally deal in air worthy parts which will not be cheap. Here is a guy parting out a 152 in FL he "has wings", you may want to give him a call and check other part out barnstormers adds.

Your best bet may be to wonder on down to your local GA field and ask some questions at the FBO or maintenance facility if there is one. Its more than likely someone has a wing or wing section that is no longer air worthy from some accident or the like. You may even just see a junked wing sitting in a field behind a hangar... 

STC's are specific to part or aircraft/engine so testing on a generic wing may not be a good basis (depending on what  you are doing) you may want to get the wing from the aircraft you intend to STC.