"Discounts" and "used parts" are usually mutually exclusive terms. Salvage yards (main source of used parts) know the market and know what their parts sell for if they were new parts. They'll price the used parts just low enough to make it worth while to buy used, rather than new.
For discounted new parts, many vendors on this forum offer discounts to forum members. You usually have to PM the vendor through this forum and they'll respond with a discount code.
For used parts you can Google salvage yards locally or for major yards try:
J&W Auto Wreckers in Antelope CA or
Jeeps Unlimited in Grand Junction CO.
Other posters can add other Jeep oriented salvage yards.
A forum community dedicated to Jeep Wrangler owners and enthusiasts. Come join the discussion about reviews, performance, trail riding, gear, suspension, tires, classifieds, troubleshooting, maintenance, for all JL, JT, JK, TJ, YJ, and CJ models!