It's bungee cord (don't know technical name). Available at Amazon and other on-line sources. Can also be made up from bungees with hooks installed sold at Walmart and Big Box stores (cut the hooks off, and cut to approximate length)
I ordered mine from Robert's Sales in Denver (https://robertssales.com/parts.html
), because one of the clamps actually broke before the bungee wore out. So I ordered factory clamps and bungee. Roberts is where I bought my A-frame, and is my go-to place for parts. Just call them on the phone and tell them what you need/want. Prices and shipping are reasonable.
2019 Flagstaff T21TBHW A-frame
2022 Hyundai Palisade