Do what my local CBers used to do. Go down to your local shopping center and rip one off the automatic doors late at night
Ok.. they are not hard to find on the surplus market and they are cheap so you don't need to steal them. Visit any alarm installer and they will probably give you a box of old units for a case of beer. If you're really hard pressed, go down to your local pawn broker and buy up a box of the cheapest and oldest radar detectors you can find and you will have plenty of Gunn Diodes and detector diodes to play with.
Plenty of Gunn heads on ebay. $10-$30 is what you should pay for them.
270897383930 270842101053 260956100537 and so on
http://www.ebay.com/sch/Consumer-El...?_nkw=X-band&_catref=1&_trksid=p3286.c0.m1538
You can also rip DRO's out of old satellite LNCs.
Klystrons are a bit of a pain in the bum to use. Gunn Diodes can be pulsed in the same manner as klystrons, but there is no need to do that.
Using code spreading sequences is a much better way to get better radar performance out of a 20mW Gunn Diode.
A Gunnplexer (tm) is a specific type of gunn head.
My webshots page is getting a workout tonight again Dave...
http://home-and-garden.webshots.com/photo/2817151550048941318Vcrkbm
That's one of the hotwheels toy radar guns that I bought from K-Mart.
In true Tim Allen style I said..."This thing needs MORE POWER... so I rewired it ! Put a 300mW gunn diode head into that bugger ! Now it starts fires at 100 yards !"
Actually, it can get speed reading off cars from about 2km away. I tested it on the Bourke Rd a few years ago.