Where do you buy
 

M3 nut's and bolts plus other thread sizes other than those packaged by the manufacturers?

Doesn't have to be just regular black steel ;)

These guys are pretty good,

http://www.westfieldfasteners.co.uk/

I use eBay.
Most sizes can usually be found in multiples of 10 at reasonable prices with free shipping from various sellers.
Boltbase is a seller I use often.

Bolt it are good

I use these guys http://www.modelfixings.com/ and have never had any problems with them.

hawks fastners in middlesbrough are good

Try Toolstation, I got 100's of M3 nylocs for like £2,

I used http://www.rc-screws.co.uk which were great!

Yep rc screws are spot on Plus they have quite a few other handy products:thumbsup:

+1 for Modelfixings - good range, good prices and fast delivery :thumbsup:

Screws & Stuff
 

I use South Essex Fasteners in Southend on Sea. Usually have what you need or a suitable alternative.