Look online for a used one or a good quality aftermarket one. If you need both, spend the extra money and get a monitor that comes with one.
If you have a monitor and need one, look for a used one online or buy a compatible replacement that is well built and tested for safety. These cost more then a generic imported one but it will be less likely to damage the monitor or burn anything down because it failed. Apple OEM is still preferred. The main problem is the adapter each Cinema Display varies. You need to know which one you are going to need before you buy one.
I don't think these can be modified to take an internal power supply, so that's probably out of the question. I think this will be the case because of how the monitor is designed. The power cord that goes to the power supply is permanently installed in the monitor so it's not easy to remove and replace with say, a IEC socket or even a standard power brick. It may still be possible but I wouldn't do it. This would also destroy the resale value of the monitor as well. It's probably easier to get an external power supply.
You can probably find a compatible supply by searching something like Apple Cinema Display Power Supply and looking for a good quality generic one. If you can't, buy the Apple OEM one rather then a generic replacement.