GOODWILL!!! it's a hidden gem. i find a lot of lightly worn and like new brand name golf polos at my local goodwill. sometimes i even find a couple club/tournament polos as well from the area.
another place i buy golf clothes from are Ross, Marshalls, TJ Maxx, and Gordmans. polos, shorts, golf pants all range from as cheap as $12-$40. I recently bought a full Nike golf rain suit from TJ Maxx for $25! and every pair of golf pants (Nike and Tiger Woods brand) i have bought from Ross or Marshalls, with the most exspensive ones being $35 (MSRP was $60+). so check these stores out frequently because they get new shipments in every couple weeks etc.