Well in my experience yes indeed buying stupid small parts (if still available) for older outboards is out of all proportion to their value and can make something like BMW car parts look cheap! I've not found any massive price difference between differing UK sources for new OE parts... but I have found a big difference between the helpfulness of dealers to looking up obscure small parts. For example my local very small & quite tatty Mercury/Mariner dealer will look parts up on his PC and let you look at the details over his shoulder so you can see what's what re availability and pricing. Charges retail or therabouts though.
Folks on here have received good service from Black Dog Marine so you could try them... Black Dog Marine. Outboard, Inboard Parts And Accessories
Used it's down to Ebay really or the rare chance someone on here is breaking the same model. Just be wary on Ebay there's a guy breaking OBs in Cambrishire who has been a heap of trouble in the past... but his recent record looks better so if he's the only guy with what you want...
Obviously all the common service parts are easier to source though.