I feel like there should be a thread just for this.
Apparently screw M3 0.5 x 14 is what I need for a Garmin Edge out front holder thingy.
I can find an M3 25 at screwfix.
Can you just saw of the end?
Is there a magic bike screw shop where screws cost pennies and not tens of pounds?
pic?
If you are cutting down a bolt, thread a nut on first, then cut. As you take the nut off it will cut the thread at the end for you.
Yes, I got recommended this place when I rounded off the stem cap bolt for my TT bike. Just took in the one I had and the guy ferreted around for a while until he found the exact one. Bought a handful as spares and cost a tiny amount.
They literally seem to have everything. All they do is screws and bolts. If screws could be classed as treasure, it would meet the treasure trove definition! You need to visit in person though, so maybe tie in with a visit into the city centre at some point?
this is why I love London
Mate, I think I’ve got a Garmin out front mount knocking around, if you need it (or the screws). Let me check.
What kind of screw, socket head cap screw? Pic…
You can buy an an assorted box of m3 screws on Amazon for about a fiver.
One of these? 99p posted. M2 M3 M4 Cap Head Allen Bolts / Socket Screws 304 Stainless Steel A2 70. DIN912 | eBay
Saw off the end.
Buy a massive Mx whatever pack for £15 or whatever it is now from Screwfix 
Sorted forever.
Or go to that place in Clerkenwell when you’re next in the office. And buy all of the spares of all bolts that you’ll ever need 
What FP said but I’d also give the edges of the cut a gentle file before taking the nut off as sawing through the thread often leaves jagged bits that you can get rid off with the file and the nut will then clean up the thread.
Ps. Have you looked on ebay, you can generally get every variety of nut/bolt known to man on there for pennies?
Wow.
I’m going to have to go around and make a list of all the bolts and screws I’ve lost. And head up there. 
Looks a bit like this, maybe an older version
Don’t have a pic of the bolt obvs.
From somewhere on the internet.
Yeah, iirc mine was silver