I think that the OP is looking for a list of airports that, with a 300 mile radius, covers most commercial airports, not just a list of all the commercial airports.

I managed to come up with this list this morning. I'm not sure that it is perfect, but it covers most of the country (the Dakotas are a bit thin) pretty well:
mht; phl; clt; mco; bhm; dtw; stl; geg; msp; iah; elp; okc; las; den; boi; smf; jac