I'm building one at the moment and have bought screws from here
http://www.spaldingfasteners.co.uk/
I've got the front end together and just put the motor plate on the gearbox.
They don't have every size required but most (and those sizes that they don't have, they have something close that will do the job). Cost around £12 including postage, and I have spares of all the screws. The only significant missing ones are the 27mm long screws that go through the gearbox to the motor plate. They have 30mm which might do, but since I didn't know how much space I'd have beyond the motor plate I didn't risk it, although I guess you could grind some down.
The M3x10mm had to be cap head rather than button head, but that hasn't been a problem so far. Also got 16mm countersunk instead of 15mm, but that didn't cause a problem at the front.
So far happy with them.