Support group for novice sewists

If the brand of waistband elastic you can get is Prym, then that’s what I use, and it’s good quality and lasts well. FWIW I’ve found this sort of elastic hard to source in the UK too — Prym was the only brand I could find when I started looking, and it seems to go out of stock more often than most haberdashery items. (I bought a 10m roll in the end, and will buy another one when this one runs out.)