Found it: https://www.reddit.com/r/headphones/comments/dg27mx/akg_k7xx_elastic_band_repair_guide_will_work_for/

I used hair elastics, based on a repair tutorial I found on Youtube; let me see if I can find it.

I have a set of nearly-10 year old Q701s (same headphones, just lime green and branded Quincy Jones)

Other than the elastic suspenders weakening (looks like yours did the same; they all do!) and the plastic sliders cracking, they’ve been great. I replaced the elastics and the pads, but I wish the sliders were easy to do.

As a Canadian, I can say with certainty: check eBay, they’re usually listed for a lot less.