Many kits come with the seat belts molded on the seat or none at all. There are aftermarket companies selling detail sets of seat belts but they do not cover all aircraft. I found a fiberglass cloth tape typically used for electrical cables that when trimmed to the proper width, can create seat belts. With a little paint and some details it can add a more realistic look to the seats. I take a piece of tape and stick it on a cutting surface and using a metal ruler trim a strip the width I need Then I measure the seat and cut the length. Since it is self adhesive you can stick it to the seat and position it as you like. Once positioned it can be painted the correct color. You can then use chrome silver paint or make the buckles out of scrap photo etch or cut pieces of tinfoil.
The tape can be purchased from Amazon at: 3M Glass Cloth Tape 361
Here are some photos of cockpits I have used this on: