Hi there! My Dunkin is taking a "petite pooches" class at the
Dog Training Club of Chester County in Exton, Pa. He's 11 months, so I believe there is probably a younger puppy class too, but you'll have to see if it's specific to small dogs. I had the same problem ... Dunkin is over 3 pounds now, and he's still the smallest in his class, but the other dogs are much closer to his size.
Also,
Le Petit Chien holds play dates, and there were lots of small dogs there!
Good luck