What is the FAA's source for the type designators they use in IACRA?
Try this.

This is the FAA's Type Certification Data Base, which lists all type certificates issued by the FAA.

Note:  this only covers certified aircraft and not experimental or homebuilts.

http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/Frameset?OpenPage