Dear me.

Theres a few on ebay.co.uk

This guy is 100% to deal with

http://www.ebay.co.uk/itm/BMW-e36-M3-3-2-Exhaust-back-section-back-box-/231201374312?pt=UK_CarsParts_Vehicles_CarParts_SM& hash=item35d4ad0068

It would also be worth trying

CC-Automotive - 07849572784

Border Motor Works - 02871 382504

Both are sponsors of NI-BMW and always have M3's for breaking.