These are some dealers that discount parts for internet orders, usually offering 20% off prices with no tax outside their state.
As usual, things change so check if they are still discounting parts if you order from them.

Oak Lawn HD... http://www.surdykeharley.com/Default.asp

http://www.tri-countyharleydavidson.com/
1-513-874-4343 Ask for Jenni. Usually 20%+ off
great to deal with too.

Chicago HD... Chicago Harley-Davidson (http://www.chicagoharley.com/shop/)

Discount Biker... http://www.discountbiker.com/

Hales HD... http://www.haleshd.com/

LP Cycles (Tom)... LP Cycles - Discount Motorcycle Parts Directory (http://www.lpcycles.com/)

M&M Cycles (Mark)...(outstanding) http://www.mandmcycles.com/homepage.htm

Zanotti HD (+ parts lookup)... https://www.zanottimotor.com/shopping/partLookUp.html

Suredyke HD... http://www.surdykeharley.com/Default.asp

Chesapeak HD... http://www.chesapeakehd.com/

Suburban Harley... Harley Parts - Genuine Harley Davidson Motorcycle Dealership (http://www.suburbanharley.com)

ohiocycles... http://www.ohiocycles.com/