For highest performance, don't believe medical latex can be beat. The material needs to be stored in dark cool place. UV is enemy of latex. It's not the most long-lived but, man, does that STUFF make a steel ball zinnnng! :-)
Primo exercise band, imo, is Theraband. Not quite as snappy as latex, but is bit more durable and maybe less costly.
Lesser band materials were avail back when slingshots were serious hobby of mine. Such as #107 rubber bands, a conveyor belt material used in mining industry (this stuff is very durable), and the cheaper exercise band materials. Don't know if those heavy rubber bands or conveyor belt materials are still avail. I just bought the low cost exercise band stuff, but haven't made any band sets yet.
The best online source, imo, for slingshot materials is SimpleShot.
https://simple-shot.com/There's also tube, instead of flat band, configurations/materials. I was not accurate with them, but many guys are.
I'll post some slingshots have made later tonight.