There are a couple of different ways to acquire new wolves:
You can breed two of your adult wolves together to get a litter of pups -- free wolves!
You can get wolves from other players -- either from buying them on the Flea Market or checking out the Lorwolf Trades subforum to see what other users have on offer. You can trade wolves (and items/currency for that matter) privately with other users through the messaging system.
You can befriend a Wild Wolf! There's a rare encounter in many areas of the campaign where you might get the opportunity to befriend a Wild Wolf. The odds of the encounter succeeding are quite low, but if you're lucky, you'll get a brand-new adult wolf in your pack! If the encounter fails, you'll get a "knucklebone token." Once you've collected 50 of those tokens, you can complete a certain Hunt and receive a Wild Wolf as your reward.
Sorry for the little delay in getting back to you! To breed a wolf, go to its page and look for a button in the "Bloodline" section (under "Details"). Click on "Start Breeding" and it will bring you to a page where you can select a mate from your den for the wolf and preview the potential offspring. Once you're happy with the pair you chose, click the "Start Breeding" button next to "Refresh" and the pairing will be complete!
A pregnant wolf must wait 3 days before being sent to a nest, and then wait an additional day in the nest before it's ready to give birth.