Cheap sources of stuff to build/modify.

In this thread, share your tips for getting the materials, supplies and tools you use to make and modify stuff as inexpensivly as possible.

A trick I use:

The local grocery store "Hardware" section is full of cheap tools and supplies. Last week I picked up a 105 piece screw/nut set in a nice divided plastic clamshell case for $1.00, two days later I saw a similar, but empty, plastic case selling for $3.99 at the craft store. The screws at the local Big-Box home improvement center would be about 6 cents a piece + the gas and agravation of driving to the store in the middle of a project because I need a couple 6 cent screws. 6 cents x 105 pieces = $6.30 worth of screws.