Where do you buy

[Chequered Flag Racing]

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

[MHeadling]

These guys are pretty good,

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

[Getpip]

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.

[mark christopher]

Bolt it are good

[i4n]

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

[Cory01]

hawks fastners in middlesbrough are good

[martgifford]

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

[coolcars782]

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

[shanks]

Originally Posted by coolcars782

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

[JezT]

Originally Posted by i4n

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

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

[CrashBangWallop]

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