Is there any possible way to "mount" the pi 2B on a PCI slot? I would like to add RGB (for whole room) control in my computer case.
It should also work if the PC is off. Therefore I considered adding my Pi in my case and power it directly via a power adapter in my PC. Therefore, having a PC in a PC.
The perfect solution would be an adapter to fit the Pi in a PCI slot. Do you have any recommendations?
Yes, you can get blank PCI cards with no electrical connections so you can use then for mechanical support. You could glue some mounting posts on it and screw the Pi on top. For power, there are two options: 1 use +5vsb from the existing power supply if it is more than 2A, or 2: use a Y-cable on your power input and add a second power supply for the Pi.
There are no models of Pi which will mount into a PCI slot without third party adapters. I'm unaware of any such adapters which are specifically geared towards the Raspberry Pi, and I'd be surprised if they existed. There are easier ways to provide 5V from the inside of a PC case.
A standard ATX power supply should provide 3.3V, 5V and 12V outputs. Find the 5V output and connect the Pi to it.
Many motherboards include internal USB sockets which should output 5V. Depending on your Pi's current requirements, it may be possible to use a Y-cable to power the Pi from multiple sockets (a single socket is unlikely to provide enough current). Exercise some caution here - it's not a particularly good idea to try and pull huge amounts of current through your desktop PC's USB sockets.
