The standards have always allowed for lots of variety. Kettlebells, suspention systems, stability balls, and steps. However, There are 1/4″ – 2 1/2″ elastic (rubber) bands that have changed my clients pull up routines forever. I posted a link but they are probably in that PS catalog. Careful, I’ve noticed Power Systems produces ‘some’ sub-standard products that are cheaply made and wear out quick. Those bands have stood the test of time though. Think I got them through perform better (who isn’t much better in the quality department, really check into what you are getting).
Guess they don’t have th 1/4′ on this site but, I got one so they’re out there.