Most people probably get them from amazon. Things like this.

https://smile.amazon.com/gp/product/B08G4NDGMW/ (nice)

and this

https://smile.amazon.com/gp/product/B09GL1Z6YJ (not sure I'd get this one again, a little too long for me)

and this

https://smile.amazon.com/Bodylastics-Collapsible-Resistance-Construction-Connection/dp/B07V6Z3171

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Some people get ankle straps.