1st thing to try is to run it through with a can of "Cataclean" - a well recommended product.
I can't see how changing to the older cat-under-the-floor system would help... the Manicat has a lambda above and below the Cat so what do you do about ditching one Lambda ?
As regards a 2nd hand one, best try a parts finder website since a few organised scrappies may have removed and stored one or two over time.
Otherwise just pay up for a new one and be glad they are available.
Incidentally mine is still going alright after 151,000 miles despite a few dodgy lambda / ECU issues.