Parts are rarely the wall people think they are. The trick is knowing which of three shelves your part lives on.
Shelf one: generic electronic parts. Semiconductors, capacitors, connectors: these come from the authorized distributors, and both big ones work for one-off repairs. Mouser carries millions of parts from over a thousand manufacturers, and authorized means genuine and traceable (that matters more every year: discontinued output transistors like the Toshiba 2SA1302 and 2SC3281 are famous counterfeit bait, and the fakes fail fast). Digi-Key's no-minimum-quantity model is what makes single-piece orders practical.
Shelf two: speaker and mechanical parts. Recone kits, surrounds, voice coils, and spiders live in the speaker-components aisle of the specialist sellers. Refoam kits are sold by four measurements, so you order by numbers instead of hope. For dead drivers, copy woofers are made and sold by the original part number for common brands (Polk, Boston, Paradigm among them). Worn rubber parts like idler wheels can even go out for professional recoating.
Shelf three: the scary ones, where you look up the cross-reference before declaring anything dead. The classic is CD laser pickups: some Sony KSS pickups are genuinely unobtainable, but the Philips CDM12 family that died of old age has documented drop-in successors. Cassette idler tires have exact part numbers by model. The difference between "unfixable" and "fixed" is often one thread that names the substitute.
The discipline that ties all three together: the label on the back beats the name on the front. The name on a faceplate is often a series, not a model. The back label's numbers are what parts catalogs actually use. Photograph it before ordering anything.
What are you hunting for? Post the part and the unit; the sourcing regulars here are good at exactly this.