I remember seeing some elastics one of our blokes had bought in an American PX. They were on a little card and had an opening link so you could put them on last of all.
Many of us made elastics from the issue braces, which were perfect. Nobody wore them anyway because they were far too uncomfortable over a long period. Plus the brace buttons inside the waistband of your combat trousers were a nuisance digging into you for over a week on an exercise. We'd just cut them off and wear a belt instead.
