Surtling spawns in the swamp biome--look for the fiery guys. Yep, not a bad source of cores if you're not in to the whole burial chamber thing or having trouble accumulating cores.

Extra pro tip: All you need is 5 cores. Build the kiln, burn up a lot of wood for charcoal, break it down then build the smelter, use coal + ore to get your ingots. Repeat as needed, until you have enough cores for your kilns/smelters/portals/etc.

As for the guy who never goes into burial chambers--it's a good source of cores (eventually) as well as a lot of other loot items. You can skip it, sure, but if you want to buy the belt or other items from the trader earlier in the game, those chambers have a lot of treasure in them.