If you are looking for the breech plug itself and not just a new drum or nipple, Deer Creek might be your only source if they even got any when they bought out CVA's stock.
None of the (aftermarket) drop in barrel builders ever made a drop in barrel for CVA's. The threads are metric, and I have never seen a (lone/replacement) breechplug for them listed at the builders supply places (Thompson Center, yes, sometimes Lyman, but not CVA).
When CVA was still making them you had to send the whole barrel to them for service if you had breech problems.
If you can't find a plug you do have other options to salvage it (have the breech cut off a tad, re-bored/threaded (or maybe threaded larger if there is enough metal), go with a Hawken style hooked breech (flint which then must be drilled/tapped and drum installed), reshape the "new" tang to fit the inlet (because a new "non-cva" plug won't fit your current tang and may not be able to be adapted, possibly trim some of the under-rib and move the key tennon) but that could cost you the value of the rifle.