Best place to buy seat belt, for '87 K20

[Chevy454]

My father-in-law has a *mint* '87 K20 Chevrolet that he's owned since almost new, and the shoulder belts are acting up...they won't pull out all the way most of the time. He's been reaching up under the plastic cover and pushing the plastic release to get the seatbelt out, but it's getting to be a pain.

Anyway, his truck is the medium blue in '87...there is another truck just like it in town, that is all beat up, that I was gonna nab the belts out of, but they do the same thing.