For those of you in Japan or have a connection living there, 100 yen stores! These are the price equivalent to dollar stores in the US, but are jam-packed with things you can not only actually use, but wonder how you ever lived without. Things like sheets of styrene, self-healing mats, alligator clips, plastic containers, all kinds of tweezers, scissors, and handtools abound, not to mention all the "I'm buying that 'cause I'll probably need it one day" items.
Outside of Japan, I've found most useful tools from electronic supply stores, craft stores, thrift or pawn shops, and rummage sales. You can find the best deals at thrift stores and rummage sales, since many items are sold as a case lot or "by the bag". I've gathered an assortment of small pliers, tweezers, blades, and other micro-tools over the years from rummage sales...there are some better quality items to be had from someone intent on cleaning out their garage or attic.