I grease my lower ball joints through the axle u-joint. You need to jack up the front end and spin the tire to align the u-joint at just the right position. You can then feed the grease gun hose from the top of the axle down through the u-joint and get on the zerk for the lower ball joint. Tricky, but easier than greasing that darn front driveshaft.
Edit: To answer your original question, I just use a regular grease fitting installed on a flexible hose.