I use a Lawsy's Mark 4 extruder ( http://www.thingiverse.com/thing:47561 ). It's simpler, easier to maintain, and more durable by leaps and bounds than the stock extruder, in addition to being fewer pieces. If you're printing a replacement extruder, print several copies of the extruder parts in case you break one in assembly. Once you take apart the stock Solidoodle extruder, you won't want to put it back together. It breaks easily and is called "the jigsaw" for a reason.

I got my 618 from Octave Systems, but it's pretty common at most of the online printer suppliers now.

618 is a little iffy with the stock Solidoodle 2 hot end because the insulator is PTFE, which could start to decompose around the upper end of 618 extrusion temperatures. Thankfully, 618 has a wide range of temperatures at which it works (albeit with a bit less strength), so you can compromise and still get wonderful results.