Casey's and HyVee has have free air. (The Grand Prarie HyVee has a cool set the digital PSI on the machine and it fills your tire to that pressure while you just hold the air chuck on your tire valve)
I recall getting free air at Meijer's in other cities.
I suppose historically gas stations were service stations that had large air compressors for their tools and a hose with chuck cost very little. Now the snack store type gas stations have no need for an air compressor to run their business.