REI specializes in niche-market radios, which frankly the entire ag market is, comparatively.
These guys make industrial-grade radios, their salesmen know their business, and you will receive the correct no-hassle plug-n-play wiring harness for your tractor if you order from them. They are not the cheapest K-Mart radios out there, but are very sturdy. I don't know if REI will sell directly to "end-users" but if not, they probably have a dealer/distributor near you.
We went with REI after we got sick-n-tired of having to hack and solder the supposed factory-fit radios from Case-IH parts depot system. The Case Ship-Direct radios and wiring harnesses didn't even fit their own tractors, but that's a different story.
Our dealership has had "zero" problems in 10 years doing business with REI. Highly recommended.
link: http://www.radioeng.com/