The cases you've linked to are Alubos 800/1000's made by Bopla. Great quality and used by loads of manufacturers for good reason (I used to have a product range housed in them).
They aren't cheap, but you do get what you pay for.
If you need a low cost extrusion, then AliBaba has a million companies making them, and they are low cost - but again, you kinda get what you pay for (wall thickness is quite thin). I've designed extrusions before, the tooling is usually sub £1K and the turnaround is short (4-6 weeks), you'll probably have to find someone to chop it up and paint it, or you can get the mill to anodise it (but better to anodise after sawing). Laser cut end panels and a stamped gasket and you're splashproof at the very least.