Bits4Vits is the only place I have found them also. Their shipping is pretty fast, or you can order them from Suzuki (for about twice as much).
Are they absolutely necessary ? No, most are not using them. If you are hammering on a lifted rig, they will help on seal wear. We ran one on our race rig, and did not have any seal issues. I don't have one in my DD lifted Tracker, and do have to replace the seal every couple years.