Not necessarily "Hardware", but I managed to craft my homemade bike light handlebar mount out of plastic Reflector holders. The bike shop let me sift through their pile of reflector and computer parts. Then I went to the hardware store for some knob-type bolts and wing nuts - also picked up some of that industrial strength velcro. Make a great mount that I can easily pop on and off the bike.
There's other useful stuff at the hardware store - 4 and 5mm hex bolts are great for replacing seat post and stem bolts, zip ties are handy for just about everything (we McGyvered a pedal back together on a ride recently with a zip tie) - The safety aisle is great, too. Good reflective material to use if you need it, and they're starting to get small LED's that you can stick on just about anywhere.
The other trick is CO2 cartridges. If you carry around the unthreaded type, you can buy the cartridges in bulk at Walmart or Target in their ammunition section. Much cheaper there than at the bike shops.