DPMS makes a parts kit with a good selection of what you would usually need, they call it their "Ultimate Repair Kit", and MidwayUSA has them for $34.99, P/N 587468. It has most of the items needed most. I've seen this kit other places, but Midway is quite a bit cheaper than anywhere else I've seen them.
A lower kit is good, too. My kit contains a lower kit, one of these ultimate repair kits, an upper kit, a pin and spring kit, and a few more of anything else I thought I might need.