I'm not sure a bike shop can legally offer any repairs on it. However, here are a few suggestions:

1. Visit a sporting good store or a (car) body shop. The straps are similar to those used to tighten loads on some racks, and similar to safety belt straps, except narrower.

2. Some fabrics stores might sell that material. But I don't know the exact strenght required (ASTM standard?) nor do I know how to test the stuff you might get at a fabrics store.

2. Buy a really cheap helmet. Remove its straps and reuse them in your helmet. Total investment: 25 $ or less

Regards,