I contacted super 73, they sent me a link for a 20 x 4 inner tube, but I found one on amazon that Is 20 x 4 1/4. I ordered that 4 1/4 inner tube as well as some tire liner to fill in the extra space (if needed).

Big-cat Bicycle Tube 20" x 4-1/4" (33mm) Standard American/Valve https://www.amazon.com/dp/B08193XL46/ref=cm_sw_r_cp_api_glt_fabc_8HXB7XPFS16W3HXRCZTQ?_encoding=UTF8&psc=1