I just ordered a belt from West Coast Belts. Mine is a Silver Reed 8700 which uses the same belt as the Underwood 565. They're almost identical machines,
via Claude Medearis at https://typewriterdatabase.com/1979-underwood-electric-565.12254.typewriter
See: https://westcoastbelts.com/product/new-replacement-belt-for-silver-reed-electric-8700-typewriter/