Velo Orange still sells them.

In the 70s 80s and 90s, any LBS would have had a box of bottle cage straps. Obviously, not any more. This is the traditional way to attach a bottle cage. Most modern cages don't have tabs that will even work with them. At one time everyone used TA cages, or REG type spring cages.

I'm using the VO straps with a Zefal reiisue chrome cage. It's OK. Looks right on a PX10. Chrome isn't as good as the originals.

King cages sells a sort of industrial looking modified hose clamp strap. If you don't mind the look, they'll work with any cage.