In Oswego, try Tim @ Heartland Outfitting. He works out of his home.
You could just do the swap yourself. If you don't have a set of punches, go to Harbor Freight in Aurora and pick up a cheap set. Then all you need is a block of wood (1x2 works nicely) and a hammer. If you have the M4 handguard you can remove it without any special tools. Put the rifle butt stock on the floor and press down on the delta ring with your fingers and have a helper slide the handguard up and swing out (I'm done his without a helper too). Installation is the reverse.
Swapping the front sight post is a 10-20 minute job.