I did that too. I was able to reassemble the gun without it, temporarily.
I found a generic replacement C-clip at Fastenal, that fit the guide rod groove. But it was too big around the outside to fit in the recess for it in recoil buffer, and the gun couldn't be reassembled with it on there.
I ended up ordering an authentic replacement from Brownells. Got a few extras, in case another goes flying.