remember, kobolds are very smart. they're basically tiny engineers who descended from dragons. read Tucker's Kobolds, for example.

and they live underground. so you've got this whole society of tiny engineers descended from dragons, living beneath the earth. they could have a whole civilization. the deep kobolds could still be serving dragons to this day. and learning magic from them.

so nothing prevents you from having one or two kobolds in your world who can cast Disintegration. maybe they're far away. maybe, as the most powerful kobolds in the world, they have a lot of demands on their time. maybe it takes a little while for them to show up.

so the party hears a big crowd of kobolds gathering. some kobold in armor gets up on some kind of platform and gives a rousing speech in Draconic to hundreds if not thousands of other kobolds, and at the end, all the kobolds cheer, and start chanting the name of a legendary kobold mage.

the kobolds start chanting this name every day.

if your PCs have temple of the gods they might have legend lore too. they might be able to use sending to talk to somebody who does. so they can find out who this name represents.

and every day, when the PCs look outside their temple, they see a larger group of kobolds than ever before. and they see the kobolds digging, and going underground with explosives, and coming back up above ground without the explosives. they see flying kobolds with little dragon wings, carrying ropes up to the top of the cave, and building a whole web of traps and bombs and weapons pointed at the PCs in their temple. the air around the temple becomes as difficult to fly through as the ground becomes to walk on. teleportation becomes the only option.

and then one day the mage kobold arrives and the other kobolds chant his name for hours. and then he gets up on the platform next to the cannon, and he starts to cast Disintegrate.