The easiest way to find them is to go to caustic caverns. Varkids can evolve into chubbies and with the the shear number of larva that spawn you have the best chance of finding one.
Thank you for the information. What I posted is what I was told on the Gearbox forum 4 days after release, by a Moderator/Exec in a discussion topic, he said they were random, but never mentioned they could be evolved into.
I have found two so far. The first one was in the creature slaughter dome which dropped an orange shield and the other was in Caustic Caverns which dropped a bunch of eridium and I think a purple or blue weapon.