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Can you purchase a bag with an assortment of nuts, bolts, washers for fitting ancillaries to your bike? Could do with a little stock pile for experementing with set up etc.
Google M5 hex socket cap bolts. Choose your own length. Remember long bolts can be cut short.
Orbital Fasteners are good. If you're below minimum postage value, but low weight - call them and they'll do a reduced price delivery for you.
Find a local hardware/bolt shop, you’ll get enough bolts, nuts and washers in various sizes and grades to see you through the next decade for less than a fiver (considerably less if you only need a dozen or so bolts)