It takes some time to learn what the most common items are, and more time to build up your own inventory. One thing that I've found is that certain sizes of nuts and bolts are more popular. 4-40 and 2-56 (and mostly the 4-40) are the most common sizes I need and use. Take a look at one of the online sources of hardware and get an assortment box of bolts, nuts, and blind nuts.

http://www.microfasteners.com/
http://rtlfasteners.com/Merchant2/me...&Store_Code=RF

If you come across the need for 6-32 or larger, you can often find them at the hardware store as well as the above mentioned sources.

Scott