I assume you’re talking about Qbert’s Headband. The strap on a gym/travel bag is called “nylon webbing”. Looks to be about $3/yd. on eBay, or you can try a local sewing/craft store. If they sell the webbing, they’ll likely carry the rest of the stuff, too. If they don’t have the shoulder padding piece, you can pick up some foam or fleece to make one. Just be sure you can get the stuff for less than the price of a bag, otherwise you might as well just “upcycle” the bag…
Some handy person should start making these and put them up on Etsy or the PVB Marketplace.
No problem. If you make one, you should do a new write-up (including sources and maybe a rough cost analysis) and take some updated/better-quality photos of the process (no offense to Qbert intended ;-))