Here is a really cheap upper to get started with:
Custom 16 Inch 1x9 Chrome Moly M4 Flat Top Upper 4 Rail FF Handguard - Special! <- you can't really build this upper cheaper.
Lowers are everywhere I have been using Tactical Machining lowers lately but they are all pretty much the same (TM has them to my door [80% receivers] or FFL [100% receivers] in 2 days.) and LPK's were $59 at midway.