crowleymarine.com has parts diagrams you can view for free, to find the part number you require for purchase elsewhere like ebay for example. Parts from the USA via ebay are often cheaper than UK, even after adding postage and customs/VAT.
ribsmarineparts.co.uk also has parts diagrams, with succession chains for obsolete/upgraded parts, contents of rebuild kits and RRP prices for comparison. It requires free registration and a browser plugin, and is a little clunky to navigate/setup the first time, but useful once you've got your head around it.
I have bought from them in the past, and they were friendly and efficient, if not the cheapest (drop-ship direct from EP Barrus it seems...)
I have also been happy with the service from Charles at Oakley Marine units: www.outboards-direct.co.uk/
for parts for my 100hp mariner and vintage Evinrude parts.
I have no affiliation with the above, just who I have used in the past.