Dave - I too have an Imasaf system which I'm very pleased with. Have you tried Martin Willems in the Netherlands and Pieces-Fulvia in France? They, along with Omicron, provided most of the parts for my S2.
Pieces-Fulvia (
www.pieces-fulvia.com) list Imasaf exhaust components on their website.
Martin Willems (
www.martinwillems.nl) list exhaust components that look very similar to Imasaf, and quite probably are, and at similar prices to what I paid from the factory.
Both companies have excellent priced websites with an English language option. Interestingly, I understand that Omicron are working on a searchable website with part level detail. That would be very useful, especially if parts are priced!