Thin metal foils are often used for larger scales - lead from the tops of wine bottles, aluminium from old paint/other tubes and from other packaging. You can buy foil from a few suppliers too.
For the buckles, apart from cast examples that some kits include I think there are some etched brass available or you can try using wire shaped around a suitable former. Although it would be hard to make 'em crisp enough you could also try using putty and sculpting them.
For slings and hanging belt with really nice effects is electrical tape (seriouly) and its really easy. Cut off a couple of inches and put the stick side down on your cutting mat and cut into strips of the width you require with an exacto and a straight edge. Gently peal off the mat taking care not to stretch or distort. Take some mineral spirits or varsol and brush on the adhesive to remove it , just a couple of swipes will do it . Dry it off and away you go. The beautiful thing with this is it has no weight is super flexible and has no memory(spring back) Really good for reins.
Cheers
Paul
Electrical tape's a bit thin for 1/16 scale Paul, no?
Can work great for 54s, especially if one wants a belt in tension, but I can't think of a common belting material that it would replicate in the larger scale.