A quick google search came up with Bill Cannon's holly farm in Brewster. Rt 6 goes thru Brewster.

Here is a link that might be useful: Brewster Holly Farm

There are no businesses right on Route 3 on the South Shore. It is a major highway. On Route 3A, you can find Kennedy's Garden Center in Scituate. From the highway you can see AJ Tomasi's nursery in Pembrooke, but you have to get off. Do these names sound familiar? Were the nurseries mentioned in regard to a specific type of plant? That might help identify the nursery you were discussing.

There's also Katsura Gardens in Plymouth which has all sorts of unusual (and often expensive) plants. It's a few miles off Route 3 at exit 6. They're not officially open now.

Claire

I was going to mention Katsura, too, if woody plants are what you're looking for. VERY good quality stock, and IMHO well worth whatever they're charging. I've spent a few hours talking to the owner about some of the different trees he grows, and he really puts his heart and soul into getting them off to a good start. He doesn't trim them en masse to a certain height, or anything else that the big box stores (and some other nurseries) do.

Claire had me worried, I was hoping she didn't mean they were closing... but the web site says 'OPEN BY CHANCE OR BY APPOINTMENT' and gives a number to call.