Try your local parts store. The following was obtained from the Auto Zone web site. Just about any decent auto parts store will be able to order you one.
RECON_I ENGINE for a 1965 CHEVROLET BELAIR
Unit Price: $874.99
Core Value*: $175.00
Part No.: 706141
Weight: 224.0 lbs.
Warranty: 18 MO
Availability: Online: Not Available
Store: Special Order
WITH 2 STARTER MOUNT HOLES ***IF COMMERCIAL APPLICATION, WARRANTY IS 6 MONTHS*** *** CALL 1-800-817-3266 FOR APPLICATION VERIFICATION AND DELIVERY TIME BEFORE ORDERING ***