When you're going for an approximation rather than an exact replica, think of the pieces that make the costume recognizable. For Indiana Jones, I think of the hat, whip,* and shoulder bag. Even though my shirt and pants aren't close to the right cut or colors, most people recognized the costume right away. Here's where I got everything:
Hat and whip - Costume set from Party City, $16.99. I'd actually planned to drive to Disneyland to get a replica hat (they sell them because of the Indiana Jones ride), but they sadly closed early the night I needed to go.
Shirt - In my closet, purchased from Target
Pants - In my closet, purchased from Charlotte Russe (I think)
Belt - Van Nuys Army Navy Surplus store, $9.99
Shoulder bag - The same surplus store, $16.99. It has a cloth strap, but I just tucked that it and replaced it with leather.
Holster - I've had for a pirate costume for a long time.
Shoes - Just brown boots with 1" heels (yeah, even 1" heels on concrete all day ended badly. You'd think I would know better).
Props - It's not pictured, but I did carry around my version of a Holy Grail. It actually looked more like one of the fake grails, but it still got the point across.