Finding barreled actions at gunshows or even barrels in bins is now a thing of the past.
I go to several shows each year and I've yet to find a barrel over the past 4 years that I would deem serviceable.
There are a few gunsmiths, that still have ''clunker bins'' that might have what the OP is looking for, but they won't be cheap.
I was on the upper floor of Del Selin's Gunshop in Vernon BC last spring and he had several ''sportered" No1 and No4 barreled receivers on the floor, as well as a few loose barrels.
The thread tenons on Martini, No1, No4 and No5 rifles are all the same diameter and will interchange on the different receivers.
The shoulders may have to be cut back or the tenon may have to be trimmed and the chamber recut, (expensive)
OP, there was a lovely drilled and tapped for scope, Pattern 14 Enfield rifle for sale at the Salmon Arm Gun show last weekend. $350. IMHO, as far as a hunting rifle goes, an upgrade. (I love Maritinis, Lee Enfields and Enfields so that is an honest opinion).
$350, =- $50 is the going rate for these rifles. Dirt Cheap at today's prices.
Yeah, I liked Del Selin's shop. Last time I walked in, someone was loudly calling another a Moron! (OK, less polite than that!), from behind the counter, and as I walked in, I spoke that "That's what I miss about Old School Gun Shops! The Personal level of Service!"
Got a good laugh, bought a few things I didn't need, and so on. Fun.
On the Barrels front, the good ones had to go somewhere. Not everyone was building a new garage or such over those years. ( a comment regarding using them for rebar)