For online genuine parts in the UK use A.J. Sutton:

https://www.yamahamotorcyclespares.co.uk/

For pattern parts use Wemoto:

http://www.wemoto.com/

As for the screen screws, just get stainless ones. Used them for the last eight years and never had an issue with a cracked screen in a fall. By the time you've factored in the cost of the plastic ones, and the fact the heads round off every time you undo/redo them (meaning you're forever replacing them), it works out cheaper to buy metal ones and just get a new screen should have an off and break it!

__________________