There are tons of stores online. If you’re in the US go to FocusAttack. If you’re in the UK Ultracabs is pretty good. I’ve got some stuff shipped to me from Arcade World UK that should arrive in a week or so. It seems like the Europeans have way more options than we do in the states!
First, read and plan a lot out before you buy anything.
So here’s what I’m doing (and your mileage may vary). The joystick and top buttons are suzo-happ competition. The front buttons (in the pic) were originally suzo-happ but I had to order different ones because of a sizing issue (that’s what’s shipping from Arcade World UK). You’ll need a control board to interface the controls to the RPi, most people love the i-pac 2. For speakers I bought the cheapest car stereo speakers and cheapest little amp I could find on amazon but a lot of people repurpose old computer speakers. Lighting is really any strip-based lighting. Mount a power-strip inside your box, plug everything into that and wire it to a single power button you mount on the back. Marquee can be ordered online. Paint. T-moulding. Plexi. Later I’m going to add in a few more buttons to do things like save state/load state within the emulators.
I’m still working out the circuit to shutdown the pi gracefully (as I’ve heard questionable reviews about the mausberry ones). The plan is to have a capacitor which stores enough juice to run it for a few minutes, then when power is cut it sends a command to the pi to shut it down. I’m fairly certain I can figure it out and there are more than a few guides online.
Just do your research, build it, and then show it off in the other forum for people’s projects. You can also get a lot of good support there.