My lawnmower is still in decent shape, and I would like to use it for several more years. The problem is that some spare parts are no longer available.

Where can I find them? Would used parts in decent condition be worth considering?

Sometimes, old part numbers are replaced by new ones, or compatible alternatives are available. If new parts simply aren't an option, used parts are certainly worth considering.

This is especially relevant for older models that still run well and do not require complete replacement.

That is always the case with rare or discontinued parts for older machinery. Finding decent used ones at swap meets or salvage yards can be easier since OEM parts are no longer in production. When you are trying to keep an older machine running, a source like this can also help you find suitable aftermarket equivalents and quality consumables for a few more seasons.