This could well be a repost, but I use this company all the time for both genuine and after market parts and I have not once had a problem with items being out of stock - so certainly something to keep bookmarked for you chaps over the pond that have trouble sourcing parts.
MP3 500 / Gilera Fuoco
http://www.gileraspares.co.uk
MP3 125 / 250 / 400
http://www.piaggiospares.co.uk
One nice thing about the site is that it allows you to pick parts directly off the exploded parts diagrams - so no part numbers needed. Just go to the reference diagram that you need and click on the part. Simple!
Obviously, you might get stung for shipping, but if you need to order a whole load of bits that you can't get access to in the US/Canada, the above sites may help.