Its probably fairly "boring and mundane" but I often use the following to add new spells to the available options:
1) The spellcaster's mentor (or circle, or wherever they learned their spells) has found evidence of new spells being used in a neighboring kingdom/land. They send the character(s) - either as the quest, or part of the quest since they're going that way anyway - to find evidence of these spells, trade/buy/steal them, and bring them back. This will sometimes cause the party to be the target of some of these new spells first, depending on how the adventure/adversary is set up. It also sets up either combat or RP as the spellcaster figures out how to approach this particular caster.
2) (This was already mentioned some upthread) Parties are always investigating ruins, in the bowels of ancient ruins are very old scrolls with information about experiments in new spells and different power sources. The spellcaster has to figure out how to transport these potentially fragile documents back to civilization, or perhaps they can't be moved, so the caster has to do research/copy/whatever the spells 'in situ'. In addition to finding them, reading them could be a challenge if written in an ancient script.
3) I've tucked torn pieces of spells, or spell fragments in the effects of enemy spellcasters, or deep in libraries/houses/temples of learning. The fragment can then be researched, finding out where it came from (or the general area), and then tracking down the remaining parts to make the whole spell. Or it has one "new spell" and hints about others. In my last game, I tucked a spell scroll in among an enemy wizard's spellbook. The spell was nothing else the party wizard had ever seen before. Of course, once the party wizard looted the spellbook, he never actually looked inside it (stupid 5e where you can just pick your spells

)
4) Extra planar entities. I've used demons/devils/whatever to pass along (or direct characters to) new spells, particularly if they're nasty spells. It sets up the character for a long term relationship with that entity, as the character has to decide on gaining the power/spell and doing what the entity asks, or sticking to whatever moral code the character follows. Lots of RP and grey areas there as the spellcaster then has to also convince the rest of the party to help them seek out this "info I got from a good source about this new power." Or the spell is given in exchange for some service or task, with some unanticipated side effects.