I'm pretty introverted too, and also just got a puppy - he's 3 months now. I have to push myself a bit to get out right now so the puppy gets better socialization.
For people socialization, I went to a couple of local coffee shops downtown (tiny little town, so take "downtown" with a grain of salt!
)with outdoor tables for coffee or breakfast. I'd bring a close friend or the hubby along, so I wouldn't have to talk too much to strangers. We'd just sit and let the world go by. Quite a few people need to stop and pet the puppy, and puppy gets to see and hear quite a variety of people, strollers, buggies, bikes, skateboards, vehicles, etc.
We've just started with our dog socialization. We have another dog at home, so that helps, but most of our friends don't have dogs either. Puppy had his 2nd round of shots a couple of weeks ago and the vet gave us the go-ahead to get out (parvo isn't really active here the past few years), so we're doing trail walks right after work when there's a lot of other people out walking their dogs. We're also doing a puppy class. Will seque into an obedience class when the puppy class is over.