The old tire size designations were very confusing. Some time back bicycle tire and rim makers got together and standardized the tire size designations, following the pattern of the auto industry. Currently tires are sized by the diameter of the seat on the rim and width, as in 35-590 to replace your old 26x1/38".

Here's a link to a chart showing the old and new sizing info.

BTW- I hope you didn't say anaything to the co-op staffer when you bought your tire. He should have known that you cannot replace a 26x1-3/8" tire with a 26x1-1/2.

Two other notes that you might miss in a casual look at the sizing chart.

1- there were two different 26x1-1/8" tires. The EA-3 or British version, and the S-6 Schwinn version. They're different and not interchangeable.
2- as a rule tires designated with a decimal ie. 26x1.375, are totally different from those with a fractional size ie. 26x 1-1/8" and cannot be swapped for the equivalent size.

By now you gather why they had to vome up with a better way to designate tire sizes.


Now for the bad news. To my knowledge there is no wider tire made for your 26", (590mm or EA-3) rim. However you can search for tires using xx-590 where xx is larger than 35. Try various widths trial and error and see if you get a hit.