Basically kill a lot of monsters and hope for a good one to drop. There's really no place where you can farm runes (besides Countess and Forge).
Self found high (nifty?) runes are extremely rare. I've found 2 in the 5+ years I've been playing this game.
Hell Countess would be your best bet for self-found low-mid runes. Be prepared to do hundreds of runs to build up to something decent. Also, be prepared with storage space (mules) for all these runes, and also chippped/flawed/normal/flawless gems for the cube recipes to upgrade them above Thul. People pay quite handsomely for Hel runes (in bulk), and Shael and Amns are useful on their own. Some people will also pay for "Spirit" runes, Tal Thul Ort Amn. That's assuming you play ladder though, non-ladder the highest you can cube to is Dol. Anything above that is sheer luck in finding them. IIRC it takes over 14 trillion El runes to cube to 1 Zod. Don't count on achieving that though...
