[slijin]

We tend to mostly use 1/4-20 hardware, augmented with assorted bits of either 6-32, 8-32, 10-24, and 10-32 depending on the application; we rarely ever use more heavy-duty bolts & nuts. I suggest you read up on the different types there are and buy accordingly. You may want to collaborate with local teams (if there are any) to see how they obtain their hardware.

Keep in mind that fasteners are not, however, limited to bolts and nuts. Other commonly used ones include rivets (most FIRST teams use pop rivets, which are much easier to use) and sheet metal screws (there is a fine distinction between these and bolts & nuts), as well as Velcro (you'd be surprised how strong the industrial stuff is).

We like to use onlinemetals.com for our aluminum, but McMaster has the fastest turnaround time, so it's worth the price if you need something fast.

A bit of stock plywood would also come in helpful, not only for prototyping but occasionally even in the final bot.