I've been looking into that as well;
I'm currently fortunate enough to still have my original 4-into-4 down pipes (with some rust & some repairs).
(Ive got some aftermarket dunstall-look silencers grafted on to the down pipes).
I'm still 'saving up'' for custom down pipes.
one place to try might be Alldens Exhausts, Market Rasen, Lincolnshire.
Maybe if enough of us want 4-into-4 stainless down pipes we can negotiate a better deal!
- thoughts...?
Andy C