I've used shin pad straps just like Monty that I had lying around for repairs in the past, also I have a habit of hoarding all old straps that are in good shape before getting rid of equipment lol. But if you're not handy you can always try a tailor or shore repair place, they'd have some elastic or nylon and can attach new velcro.
Here are two examples of my amazing sewing skills (insert sarcastic voice there)... pair of U+ CL elbow pads I love that had this web sort of strap with a "non slip" coating that wore off and started to disintegrate, I cut that out and added two straps, one was a shinpan strap (which I later shortened even more), and the larger was a part from an old neoprene knee brace.
Untitled by Jon Schusteritsch, on Flickr
and this was a shoulder pad straps that was way overstretched, I cut it to just a few inches and attached part of a strap I took from some arm pads we removed from my son's lacrosse gear:
Untitled by Jon Schusteritsch, on Flickr
I will use anything I can get my hands on that will do the job lol