I would definitely buy one. It would probably be faster, and it's quite cheap to accomplish these days.
Exit Granite Citadel into Tasca;s Demise - head north and east. Mineral Springs is at the N/E corner of Tascas. As soon as you enter Mineral Springs turn left and head N. The cave, as has already been mentioned, is in the N/E corner of Mineral Springs.
The fastest farming build would be any of the perma shadow form sins since the only things that can touch them between Granite Citadel and the cave are the bird archers. (be sure to take a running skill to avoid them). Just run to the cave, kill, map back to Granite and do it again.
The drop rate is low - Took me 8 runs to get one.
As to cost - they advertise for about 15k but you can usually talk the seller down to about 12k.
It can be done in Iron Mines of Moladune (mission) Ring of Fire, Abbadon's Mouth and then lastly Mineral Springs. (I think it's actually the bonus in Ring of Fire.)
If you're going to go farm there, make sure you *don't* have the Defend Droknar's Forge quest active. If you do, the cave will be crawling with titans and all the imps will be gone.
I know IDS is popular but i don't see them being sold in town anymore maybe there are better swords out there either that people cannt be botherd to farm thse imps anymore