I wouldn't go to the LHS. Go to somewhere like Fastenal or McMaster Carr and get some M3 hex cap screws. I usually pay about $4 for 100, where you might pay $3 for 8 at our LHS. I bought bulk M3 screws in different lengths, machine thread I believe.
If you're scared to buy your own hardware, you can go with this kit.
:::link removed, fast eddy now carries and those other guys suck more than Eddy sucks:::
You should be able to look in your manual and find out what size everything is, then go to the supply center or order it from mcmaster car. For the cost of the above kit, you could probably get about 1500 pieces from a supply center.
In a pinch, Lowes and Home Depot and the hardware stores sell them, but usually they only have the phillips head stuff around here, and I work hard to get rid of all the phillips head stuff I can.
If I were you, I would go to the hardware store or Fastenal and get a bunch of 3mm nuts to replace those plastic ones. Even if you buy the screw kit, you'll learn that you need extra everything when you start competing. Nothing like working on your rig at the comp and dropping a nut in the grass (thats funny lol) and not having extras.