This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
Sure. Check out the power supplies listed in the MPJA, JDR, Jameco, and similar catalogs (see http://home.swbell.net/rubywand/A2FAQs3VENDORS.html for links). Mainly, you are looking for a relatively compact switching power supply which fits the following guidelines: 1- all four Voltages (+/- 5V and +/- 12V) are available 2- the power supply can be fitted into your Apple II 3- the +5 Volt output is rated at 5-10 Amps 4- the +12 Volt output is rated at 1.5-3 Amps 5- price is no more than $25 As more and more new surplus power supplies are dumped, part of your shopping challenge is finding one which is small enough to work well with the Apple II. This explains the price guideline. If a unit costs more than $20, the odds are pretty good that it's too big physically or electrically. Too big electrically? In general, power supplies need to be loaded in order to do a good job of regulation. A 25-Amp output which is called upon to deliver 1 or 2 Amps is more likely to exhibit problems with feedback and regulation than were it asked to deliver, say, 5 or 6 Amps. An Apple II with a few power-sucking Slot cards will work fine with many PC power supplies; a base system with no cards could have problems adequately loading some PC supplies. Your best choice for an in-the-case replacement is likely to be some $15.95 unit rated at a total of less than 125 watts which was originally designed to power a printer, monitor, or portable PC. Probably, it will be "open frame" with no case, switch, fan, or power cord. Installing a power supply unit-- i.e. one not specifically designed for the Apple II-- inside your Apple II can involve a fair amount of work. When I did such an install on our II+, the most time-consuming part was fixing up a way to mount the On/Off switch and AC cord socket in the usual power supply opening in the back of the case. (Mounting the stuff on a small plastic panel set into the opening worked fine.) The power supply board itself mounted nicely on spacers in the space provided for the standard PS. Once the AC and various DC lines were connected, the new PS came on-line without a hitch. Our II+ hasn't had any power supply problems since.