The Fish Nook in Acton lets you buy any piece of LR from their displays (no one else has let me do this) and some of it is just covered in corraline - that is where I got my first pieces. Prices are very high for their rock though $12+/lb (maybe for good reason). They are on the small side but have a solid selection and clean tanks.
Unique Aquaria up near Tewksbury has nice LR (not a ton of corraline) for good prices $5/lb or $3 for rubble. They have tons of gorgeous coral (many frags), some really cool fish, and tanks are pristine.
If you really want to go for a ride, Jays Aquatics up near Salem NH has a lot of what seems like man made corraline covered rock and some more natural stuff with less all for pretty good prices, around $6/lb. They have a nice coral and fish set up.
Tropic Isle in Framingham has a good variety of rock but none of it is particularly corraline covered and I am somewhat skeptical of the upkeep on a lot of their tanks (I have seen a lot of dead fish in there).
Uncle Neds in Millis is closer to you but I have not been.
Not sure if that is what you were looking for but I hope maybe it helps. I am a rookie at this yet so you don't want any of mine...
