The hardest part I've ever seen is getting the original one out. If that's your problem, just use a good pair of vise grips.
The replacements we've always used were from Kirbans. I don't know if they carry them still, but I'd bet G Body Parts would have them. Call them as everything they carry hasn't been posted to their web site yet - 717-359-9692
Heh heh, he said he lubed the o-ring, heh heh...
I'm pretty sure he got the one from Full Throttle...same as I got. It was a very tight fit. I had the advantage of having the motor apart when I installed mine. I could probably sneak away on Sunday to lend a hand... We gotta fix that suspension issue too.