Actually, finding someone to sell you isn't that hard. It's finding someone you trust to ensure that the mag is pre 1994. THAT's the really hard part. It's times like this that good people like Carl at FSGuns is a real good resource to have.
When you are facing a potential $10,000 fine and/or 10 years in prison for EACH magazine, you can't be too careful.
And JonJ is right, most of the most reputable dealers you might trust won't sell to MA. King Tom has them all scared. Amazing what the unlimited power of government can do. Too bad nobody in this state cares about gun owners. Any other product would have him strung up in a tree for abuse of power.