Specifically the 2.5mm bolt from a Chris King ISO rear hub one piece axle lock-ring?
The travails of a cack handed home fettlist eh!
I "think" the small bolts that secure lock-on grips may be of use to you, only of use if you have an old set of grips lying around though
ebay is good for small bolts in small quantities. A2A4 are good for stainless or ProBolt if you want something more exotic.
if you're anywhere near guildford then
They do have online as well, assuming you know exactly what you need.
Just for ref, old rear King hubs that bolt was actually an imperial size (3/32 I think), they later swapped to 2.5mm on the rear, 20mm fronts were always 2.5mm I think. (caused a lot of people to then round them out because they were using the wrong size wrench on new hubs). Anything from the last 10 years or so should be 2.5mm but there are a lot of old Kings still going so worth mentioning, and apologies if you already knew this!
How come you need a new one, lost or broken? that bolt does not need to be very tight at all, and I'm forever telling people to go easy on them, so when you do get a replacement get/borrow a torque wrench and get a feel for what 10in-lb / 1.1Nm actually feels like, you really shouldn't be rounding or stripping that bolt.
Try BAPP for bolts, they stock everything you could possibly need, I'd be surprised if they didn't have this one?
Try [url= http://www.westfieldfasteners.co.uk/ ]Westfield Fasteners[/url] as well - you can order any quantity from 1 up, priced accordingly. £3.50 postage but sometimes works out cheaper than ebay.
Alternatively, let me know the exact spec - diameter/length/thread pitch/type (SHCS?) and I'll post you one if I've got one.
[url= http://www.bbn.co.uk/ ]Bradford Bolt and Nut [/url]should be on your contact list.