Had a nice seneca .36cal many moon ago for the wife, found a douglas barrel in .45 same width flat to flat, and was a little longer so got a correct breechplug and had it fitted , added sights , had 2 rifles in one then !
There's at least two .45 cal barrels F/S on E-Bay, a 24" Cherokee bbl @ $100, and a 27" Seneca @ $200
Both will drop into your Seneca stock, but the Cherokee barrel may/may not need a little reworking due to it's RR thimble positions vs the Seneca thimble positions.
It does seem that any decent Seneca is north of $400 these days. I'm not sure that Green Mtn even makes a barrel for them anymore. You might try posting a want ad on this forum.
I bought a .45 Seneca in the late early 70's and shot it for years. I was looking though EBay several years ago and found a .36 barrel there for less than $100. There are still good deals out there. Bill