A popular place to find 4BT's is through
Dovebid.com.
Look for their transportation auctions. When the time is near, you can view the auction catalog for 4BT-equipped trucks. Seems like most of these are trucks from Pepsi and Frito-Lay.
Buy a P30 van, and take what you want. Scrap the body, maybe sell other drivetrain components... you end up with a pretty cheap engine.