My experience has been that all tires for old three speeds got harder to find with the pandemic, then has gone through cycles of easier and harder to find as the various countries of origin lock down and start back up, and repeat. The more readily found ones are 26 x 1 3/8 (590mm), followed by the 28 x 1 1/2 roadster, and then the most challenging is the Schwinn (also called 26 x 1 1/4 also called EA1 and 597mm). All prices have gone up somewhat since 2019, but some more than others, and the prices cycle with the supply. Last I checked, a few weeks ago, you could get all of those sizes online, but nothing was truly "cheap".