Try https://www.fowlersparts.co.uk/ – they’re great for sourcing all kinds of genuine Yamaha parts.

When on the centre stand the rear wheel may rotate a little with clutch lever fully engaged, but not with much force. I.e. you should be able to stop it with a light press of your boot. When off the centre stand, do you feel the bike trying to creep forward when the clutch lever is fully pulled in? Is it worse when it warms up? Do you struggle to find neutral OK with clutch in and engine warm? If so, you’ve got clutch stick. This can be incorrectly adjusted clutch cable or incorrect specification clutch springs or plates fitted.

I tried some ‘heavy duty EBC springs on the clutch once, they were terrible, springs far to strong such that once warm I couldnt keep the bike stationary with the clutch fully engaged, to the extent I needed front brake fully on and it would either stall or do a burn out! I was resorting to kill switch at every jucntion I stopped at! read more here: https://www.beginnerbiker.com/2017/06/fazer-clutch-service/

If however, when warmed up you can pull the lever in and the remains bike stationary with no effort/brakes, you’re OK. Your clutch lever should freely move 10-15mm (measured at end of lever), before you feel resistance on pulling it in. No free play here could lead to clutch slip when accelerating hard, giving you a sudden surge in revs whilst not going anywhere.

Hope that helps!

Sprocket case cover gasket is part: GASKET, CRANKCASE COVER 1 (3HE-15451-10)
£18 from Fowler’s, new genuine.
The clutch case cover gasket is part: GASKET, CRANKCASE COVER 3 (4TV-15462-00)
£17
https://www.fowlersparts.co.uk/parts/4181554/fzs600-fazer-5rt4-2003-070-a/crankcase-cover-1

That said, the sprocket case cover gasket is less critical, it’s doesn’t form a seal (it’s open one side anyway for chain!) Some have noted issues with the thickness of the gasket affecting the clutch push rod going though. It’s small however, probably be taken up by adjusting clutch cable slack. If I recall, I did replace it once, but when servicing in later year went without it. You could always get some generic gasket card cheap and cut out your own gasket to fit here.