Like Neal said, looking on eBay as well as Oldgas is a good idea. The problem is many sellers don't call them ad glass, but refer to them with other words. That makes it tougher to search for them efficiently. You have to be patient.
You can always go with repro ad glass while you are on the hunt for originals.
Click on the image to see ad glass offered by Shop Talk sponsor/advertiser Time Passages. Occasionally vendors may have originals in stock, you have to ask.
While you are looking, take care that you get the correct size. There are several 12 x 5 ad glass versions.
Size 10 - 5 by 12 1/4 Flat most Gilbarco pumps and Tokheim Models 36 and 39 front and rear
Size 11 - 5 by 12 5/8 Flat with chamfered corners Wayne Model 70 front and rear
Size 12 - 5 by 12 5/8 Flat with used with most Bowser pumps
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Jim "Oldgas" PottsYour host and moderator