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I’m talking worldwide (I only have first-hand experience about Finland, but they seem common according to Wikipedia)

> maybe it's just the post offices around where I've lived that don't bother selling the "temporary" ones.

Are we talking about US? If so, it might be your office. The one near me tries to sell me all kinds when I actually go in to do stuff. USPS still sells varying rate stamps ranging 1 cent additional postage to $30 priority mail express stamps.

The most common UK First- and second-class stamps (the ones without a price on them, just a "1st" or "2nd") don't expire.

Of course you don't need a stamp, you can purchase postage online.

The monetary ones (1p/2p/50p et al.) don't expire either, you just need to meet the cost at the time you use them. (Exactly like cash in that sense: loses value with inflation, but you can still use what you have, just for less.)