**broken link removed**
http://www.thinfilm.com/
if you're looking to make a filter or some other circuit, you can use Alumina as the substrate, but you don't want to make the whole thing with alumina. It's brittle and will *****, so generally it's smaller things that are done on it, not full boards.
IMA's are usually custom. start with a block diagram design, buid up your circuits (generally bare die amps / attens / pin diode switches / and filters (alumina / rogers)), then build up a housing to enclose the circuits (brass works well for protos). Don't forget to put walls into the housing to channelize the different paths and keep a lid on it to keep the radiation inside and inside the channels.
learning really just comes from doing, on the job. I worked in IMAs for a long time, now I'm doing PCBs. They are two totally different ball games. Got any tips for me?