Where do I find these parts??!!

The clips that you use to adjust your stirrup lengths which are a metal material, where could I find them? I lost one when I was switching stirrups now I can't find it! Thanks

Are they Blevins buckles? You will need to know what size you have and whether the posts are horizontal or vertical.

Here is a site that offers the most common sizes in either all metal or leather covered.
Weaver Leather, LLC - Blevins Buckles