[Daedalus]

Adapting an ATX PSU is probably the most common method. There are no plug & play options that I know of, but Ian Stedman's adaptor is pretty close. I would try to get a good quality, low power rating ATX supply however, as the higher performance PSUs tend to have higher minimum load requirements, which an A2000 might not meet. A small form factor PSU will also be easier to fit inside the original casing as well as being available in lower ratings.

Yep, going over the rating rather than under is the way to go if you don't know what the system actually uses, because a circuit will only use what it needs. But going too far over increases the risk of problems with minimum load that I described above: a stock A2000 only uses around 2A on the 5V rail, fitting a PSU with a 30A 5V rail could be problematic unless the PSU is designed to also handle such a low load. Older PSUs are better at this than modern ones.

The A2000 also uses practically nothing on the 12V rail as standard, and most current ATX PSUs treat the 12V rail as the primary output, expecting quite a heavy load on it. So even though the A2000 PSU might be rated for 8A on the 12V rail, 6A or much less will be plenty unless you plan on filling the machine with very old hard drives.

Another option is to use an industrial PSU module inside the A2000 PSU case. These are available in much lower power outputs than most ATX PSUs, and are more suited to Amiga use as a result. However you need to know what you're doing as there isn't a standard adaptor available like there is for ATX.