I've been looking on Ebay, and have found some options. Any other places I should look? I'm shooting for 400-500 hp.
GM performance
they can get you many options at varing prices
and you know it's done right from GM, not jimmy joe motors
427 Small Block Chevy Crate Engine With 550HP. Built By www.proformanceunlimited.com For More Information Visit Our Web Site Or Call 1-800-267-3940.
This is the engine crated and ready for despatch. We're hoping it'll get here before Christmas (some present!).
Live run of a Chevy 427 with 460hp crate engine....check us out on the web at www.proformanceunlimited.com.. ..Where The Proof Is In The Proformance
Large Efficiency Crate Engines. Get a brand name new entirely-assembled engine that's almost ready to run and delivers traditional efficiency
Substantial Overall performance Crate Engines. Get a brand new fully-assembled engine that is virtually ready to run and delivers classic overall performance
Substantial Overall performance Crate Engines. Get a model new totally-assembled motor that's almost ready to run and delivers classic performance
355 SBC ENGINE AND 350 CRATE MOTOR (Topeka) - $1: 355- FLAT TOPS, 9 TO 1, X-RODS ARP BOLTS, SOLID 105 CAM, 76CC ... 
Street: STREET CRATE ENGINE-NEW 350 SBC WORLD PROD for sale 
Chevrolet 400hp 350 SBC Street Turn Key Crate Engine: US $7,200.00 End Date: Sunday Nov-06-2011 7:37:38 PSTBuy I...
Chevrolet 425hp 383 SBC Street Turn Key Crate Engine: US $8,300.00 End Date: Sunday Nov-06-2011 4:07:55 PSTBuy I... 
This was my "Fleet" of cars back in summer of 2005.
Higher Functionality Crate