Clasp for flash holder strap.

[caig]

In the UK they are known as garters. Most shooting and outdoor shops stock them both with and without the flashes.

[fluter]

In the UK they are known as garters.

Here, too (at least by Boy Scouts, though they wear stockings instead of hose ).

Jay, I think Sport Kilt uses those fasteners on their tartan flashes.

[slohairt]

All of the flashes I have bought over the years use Cummerbund clasps. (Which yours could very well be a different looking variety!) You can buy them at a fabric store for less than $2.00.