There are two options.
1- if you have metal shoes with slide in pads, there may be pads that fit, and that's the least expensive option.
2- you can replace the entire shoe as long as the mounting hardware matches the brake. If you have a road bike with caliper brakes, a 6mm slot was pretty much standard back then, (and still is, though less so) so just about any shoe with 6mm hardware (may be marked for Shimano) will fit. You have a choice between one piece molded shoes, or metal holders with replaceable pads, which cost more now, but you can save later (not a smart decision if you plan to wait another 30 years).
Kool Stop and JagWire, both offer top quality replacement brake shoes.
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