Three nice antique malls in Galesburg, just off I-94. One is even an old Sinclair station and usually has some petro items. There is another in Coloma, just off I-94. Can see from I-94. I have made some petro purchases from all of these over the years. Also, you might try to stop by "Done4". Les just lives off I-94, at the Union Pier exit, and you can spend a lot of time looking at his great collection.
As "Robert77" stated, Gilmore Car Museum is great if you only stop to see the reconstructed original Shell station.