All my PowerMac G5s have had at least one replacement optical drive. One has had four... In all cases, the first hint of a problem was incorrect reads (or no read) of known good CD/DVDs. I've been told that it has to do with plastic lenses and laser light but that's only hearsay.
I've gotten most of mine from Other World Computing, though I've gotten a couple from Amazon when I knew the exact model.