I saw a while back that Jim "Whiz" White had sold one here a while back, but I think he purchased it from the guy (can;t remember his name, maybe Marcello?) in Italy that makes or obtains some Perazzi parts.
Still confused, the barrels are inserted into a solid block of steel (monoblock) and brazed or soldered in place and then mounted onto the receiver by way of lugs or hooks. If it's a momeblock than that's a machine shop item, no factory or gunsmith for that matter would sell such a high liability item as a separate item.
Briley may be a source but they would want to make the attachment because of the aforementioned liability.
I don't know of anybody, manf, machine shop, or gunsmith who stocks or sells monoblocks.
To Truthseeker - you used the correct term. Monoblock is indeed (as Dead on 4 indicated) the hunk of steel into which the barrels are soldered. Please, again....why do you want a monoblock. I can't imagine blowing up a monoblock but still have barrels to fit into a new one. Also, if its a wear related issue, they can be welded up for worn away metal and reshaped/refinished. So, what happened?
When Stan Baker was alive he supplied the mono blocks for his Big Bore barrels for Perazzi's. I don't know if he had a source or if they were made especially for him, they were plain no markings of any kind on them. I also know that he could not obtain mono blocks for Beretta O/U barrels, you had to forward complete barrels to him if you wanted a Big Bore barrel for a Beretta O/U and he would modify and use the existing mono block.