Hello. You can sometimes get them them in the mole market when you claim the bonus, you can play the arena and when you win you can get tix to buy them, you can get them during special events - the Murkwood event just finished a week or so ago, and you can purchase them from the flea market. I have a few extras I can send your way. ^^
Adding onto what Wolfster has typed; You can also get companions during the weekly events, so this week is cooking. Under Shops > Cooking Camp you can purchase some companions with the cooking vouchers, every week these events rotate so there's loads of companions you can get with vouchers from the weekly events.
The vouchers stay in your inventory as well, so don't worry if you cannot afford one right away!
Edit: Unfortunately Gauntlet and Campaign does not drop companions- There are a couple of encounters in the Campaign where you can get companions, but usually you need to give items for them.