Goldfish Connection offers some exceedingly nice fish on auction every month or so. I would recommend them far and above anyone else as far as purchasing healthy parasite free fish.
Arthur (goes by the name Chzang) over on
Aquabid is also widely acknowledged as selling quality fish. I have fish from both places and the quality just can't be beat.
Everyone else I view with skepticism. These fish are imports- not US bred, and fish MUST be quarantined and treated for gill flukes and internal parasites, as well as a host of other problems.
Joining a local Goldfish/Koi club can be a great resource for fish as well as help when things go wrong. And with Goldies things
will go wrong. I find them to be more difficult to keep healthy than even Discus or the toughest marine fish. They are the one animal on the planet that has suffered the hands of man via selective breeding for the longest periods of time, and their health certainly reflects it.
Puregold is a great resource for info but Ingrid is a cranky old bat and very set in her ways with no room for anyone elses views.