Hi and welcome to forum.Sure others will be along with more info,but you could do worse than contact Here .Mainly Volvo but keep good range of Mercruiser stuff.Failing that try Keyparts you stock "Sierra" pattern parts at sensible prices.
Rusty, Hawkins on the Medway stock mercruiser as do Boatquays who go under the name of stapley marine as in old Ray Stapley. If at all possible I use keypart as they are well priced and normally turn up next day.
The mec cruisers are basically GM truck engines, marinised. I used to get a lot of parts form Riddley motors, importers of American cars. Sorry dont have contact details any more. But provided its not the marinising bits you are after its worth a try.