My number 1 DIY principle is to make what is simpler and cheaper to make than to buy in a store. That being said I saw these in a fabric store. I paused because they claim to magically keep your pants in your boots. I inspected the package anticipating just that... magic! Instead I found a piece of elastic held together by Velcro. In essence, a rubber band you can open and close.
Total DIY for $3 max. This is being sold for $11.99.