The Boot Garter
Furiosa wears a custom-made brown leather boot garter over the top of the left boot. There are three adjustment straps (roughly 5/8″ wide) with brass centerbar buckles, and brass keepers. The buckles are the same style as used on the straps that hold the Immortan Joe emblem onto her belt, but smaller.
For a similar product, try searching eBay for “double restraint cuff gauntlet.” It is a large black pleather restraint cuff.
The Right Boot
Upon close inspection, it appears that Furiosa’s right boot is not covered by a gaiter or half-chap, but by the right leg of Furiosa’s pants.