Sources for high quality lamb at reasonable prices
You can't beat the halal butchers around here for quality and price. They may not have baby-size however, but it won't be old. There is Al Hoda in Central Sq Cambridge, the place in Haymarket, and a bunch in the East Boston/Chelsea/Revere/Everett part of the area. They are generally local lamb, usually from New Hampshire.
I think Boston Lamb and Veal is where a lot of restaurants go, but it seems wholesale only.
went to puritan beef yesterday on blackstone st., in the haymarket area. butcher was happy to take me into the walk-in where they had 3-4 whole lambs, plus other critters, hanging. leg was $6.99 pp and they were happy to grind some for me. had a bit last night and it was quite delicious. with whole sheep there, obviously you can get whatever cuts you need.
also had all sorts of goat cuts and off-cuts of beef, like tail and tongue.