I ask for used cassetes at photo labs; they usually come with a short leader (3-5mm) of film sticking out of the velvet.

Instead of attaching the bulk film to the spindle, I just tape (with regular transparent tape) it to the leader sticking out. No need to dissassemble the cassette and/or buy reloadable cassetes.

The photolabs usually give me a bag of 30-50 cassetes every time I ask them. Looking at the brand/type distribution, 99% of the films processed here are kodak consumer, with one or two Superias thrown in. Never found a slide film cassette.