At one point, Wal, from the Beretta forum, made a 9mm barrel, to fit a .40 slide, but full size only. The last one I believe was sent to Mr. Langdon.
Normally, it would be cheaper to find a separate gun, now may be the exception to that rule, however even slides and parts are being sold for more, based on what I see. Patience is key.
I've been watching out for a .45 PX4 and recently found a police trade in for under my budget. In another year, we may see .40's being traded back in.