I've had some luck finding original bayonet-fit lens hoods, both for Pentax and Tamron lenses, by doing a general search (not just eBay) for the specific part number of the hood. Some retailers maintain stock lists of "odds and ends" whilst some 3rd-party manufacturers don't list on eBay. However, just because they're available doesn't mean they're necessarily cheap!
The original part number for the hood, should be in the associated documentation for the lens … which may be almost as difficult to find!
If a lens doesn't have a bayonet fit for a hood, any suitably-threaded rubber, metal or plastic hood of appropriate length will serve adequately. If the lens is being used on an APS-C body, rather than full-frame (or film) it can afford to be longer than the original.
I appreciate this isn't a solution if one is trying to complete a collection