The first one is how to make aluminium frontpanels with "rubber holes" for buttons and text - shown on the attached picture. What is the technique called and can it be done on plastic enclosures?
Second is where to find cheap plastic enclosures in larger sizes - like this 220 x 145 x 75 mm. I think enclosures from Digikey or Farnell are to expensive - offen more expensive than the electronics within it.
Is silk screen the word you are looking for? Not sure about enclosures though...how expensive is too expensive? a 22cm x14.5 is going to cost quite a bit since it is so big.
It's a bit hard to tell if the button markings in that panel are holes or are a solid piece of plastic/metal.
Approximatly that size case costs 40 dollars on Digikey. Considering you're getting a heavy duty case it's not that much. If you need cheap you're going to have to actually go cheap, thin plastic that doesn't protect very well.
I remember seeing, at a robotics competition in Korea, a controller housed in a tupperware container. Must have cost all of three dollars, but it did look good.
I'm building a power supply
Is it really right, that power supplies have to be built in metal boxes?
Where can I get earth in the power supply - the metal/plastic box?
The mains earth connection could be connected to the metal box.
A pastic box doesn't require an earth connection.
For most applications it's better if you don't earth the secondary side of the supply; however it might not be required if the transformer doesn't have sufficient insulation between the primary and secondary.