You've got essentially three options.
You can make your own from tape or foil (the stuff off a wine bottle is ideal)
You will have to bend some fine wire to represent the varios catches and buckles. It's doable at 1/48 and 1/32,just about. Most of us mere mortals are really going to struggle in smaller scales.
You can buy photoetch belts,often pre-painted,for all major scales. They come in some high end kits as standard. They are an easy enough option,you really only have to bend and glue them in.
You can buy belt sets in which you will get all the little photoetch buckles etc and,usually,pre-cut lengths of paper belt material. These are tricky to assemble but,particularly in larger scales look the dog's danglies.
This is the method I most often use.
Here's a set from RB productions (Radu Briznan)
They are the late war,green "Orlon" belts fitted to some Luftwaffe aircraft.
Here are the assembled shoulder straps to give you an idea of scale,and what a fiddle they are,even at 1/32!
Here's some traditionally coloured Luftwaffe ones,unusually over a pilot
Cheers
Steve