Pull one off and measure the width and diameter you need. The give PST (Performance Suspension Technologies) a call. They have all kinds of urethane bushings that will probably fit but, you may have to use something from a different application. You might also try UMI Performance...they build drag car suspension parts and have bushings for just about anything. The hardest part may be getting a person to work with you over the phone to track down the right size bushing. Nowdays...if it ain't an off-the-shelf item that can be looked up in a catalog...then you're usually out of luck. However, if you do track down the right bushing...post it up!
I'm guessing 70s GM trucks might be very close. Might also try full-size Jeep (wagoneer) or CJ parts. I'm sure its out there...just gotta find it.