There are several quests that quietly reward companions, but none will “drop” from defeating enemies. If you don't want to buy from the mole market, you can find more companions for sale in the Arena Shop, the Flea Market, and the weekly event shop.
They dropped during Alpha and were shown to drop on the site for a period of time before EA, but they did not drop during EA. (with the exception of the Mutterbirds, which couldn't be fought in the campaign and was eventually fixed)
Honestly I really wish we could get companions from campaign like we did in Beta. There's not even a bestiary so they aren't really useful for complexionist purposes. I'm not gonna spend 55000 peebles for 1 companion as much as I like it..