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What sort of head are you after?
http://www.clerkenwellscrews.com will no doubt have something which would work and you may be able to go in person if you're in London?
Otherwise as long as it's not something special head wise then Halfords/B&Q/Wickes/Maplins/etc. usually have smaller packs like so: http://www.halfords.com/workshop-tools/garage-equipment/fuses-electricals-fixings/halfords-socket-cap-screws-m6-x-20mm-hfx253
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Search ebay for either HTS or black bolts
Here are some HTS ones.
http://www.ebay.co.uk/itm/M6-Black-Allen-Key-Type-Cap-Head-bolt-Setscrew-Metric-bolts-XHCH620-10-pack-/151639697287?hash=item234e6e2787:g:N4kAAOSw4shX4oci -
where do I get ones that will be magnetic?
Everywhere. If you can't be bothered to go outside, search online for 8.8, 10.9 or 12.9. My favourite shop has 12.9 (the strongest grade) for 6-18p each depending on length with a MOQ of 10
I am after some m6 bolts to use with hexlox and they have to be magnetic but all I can find online is stainless .. where do I get ones that will be magnetic?