If you already know what you're looking for, you can go to HomeDespot and save a dime a stick.  I (shamefully) do.  If you don't, look in your local Yellow Pages for drywall (sheetrock) places, and go there.  Tell the girl at the counter, 'I want four sticks of regular plastic cornerbead.'  If you spend more than five clams, post me and I shall reimburse.  By the way, this is also the place to get 1" and 1½" styrofoam sheets 2' x 4' at the best price.  This is where I get the stuff, and cut up on my saw to fit the box.

Once you do this, you can of course then go to HomeDespot and save a dime; but like me you will probably go back to the drywall place to repeat, both for your industrial ethos, and to flirt with the sheetrock girl     :^)

Serious, in gold-plated Maui, the stuff is $1.08 / stick (8').

Prosper,

E.