Trying half.com or ebay sounded great. What did you look for? "Script" was too general. "Play script" or "theater script" didn't quite work either.

Powells is really great! I loved it when I lived in Portland, and now that I'm 900 miles away, I still shop there. They deal in used books as well, and the website will tell you exactly what condition they are in, and they ship them to you for next to nothing. They will even search for hard to find and out of print items for you, so if there's a play in mind that you can't find there, contact them about it and they'll let you know when they find it! Not to be a walking ad for Powells, but it's a great way to go.