The bolt and washer is there to stop the ball from lifting out of the socket.
The hex head bolt (or set screw) is an M6x16mm and the washer is an M6x14x1.5mm form C.
EBay is your friend here.
M4 M5 M6 M8 M10 'FORM C' FLAT WASHERS MIX POSSIBLE TOO !
www.ebay.co.uk
Beacuse you are going into alluminium it's advisable to get BZP components rather than stainless.
There is little force on the bolt/washer so no need to over tighten.
Pleanty of waterproof grease (Morris K99) slathered over the ball won't go amiss either.
Adjusting the spring pressure on the gear shift is one of the most noticable things you can do, it makes the gearchanges really schweet..