Most companies use an OEM (original equiptment manufacturer) to make their keyboards for them. iRocks, iOne, and Costar are all OEMs (WASDkeyboards, CMstorm, and filco all use Costar as their OEM) then they just sell and distribute them.
That's the norm for retail keyboards (and retail items in general)
Then there's DIY keyboard projects. most of these have cool features like aluminium cases or programmable keys. For many of those, you just need to get the "kit", then add keycaps, diodes, LED's resistors, and ofcourse keys (and a cable). Its common for such a kit to contain just the case, the PCB, feet (if necessary) and screws.
Not always of course, In some cases the PCB got designed and released, and people made their own cases afterward. The A87 is an example of this. I think the mxmini is too, but I don't know that much about it.
If all you need is the PCB, there are some places to get it. Usually there is a group buy for them (like the phantom). Sometimes you can buy it from a place (REDLINE might have some more tRiK PCB's left) etc.
