Without knowing what you are planning to use these for, it's almost pointless to discuss.
Obviously PC power supplies put out an amount of voltage/amperage to rails of different ratings. 12V/5V, etc.
If you are looking to do some science experiment and don't need all the voltages, purchasing a step down transformer, or AC/DC converter, etc. are going to be cheaper and more reliable.
If you are doing a PC, even a cheap one, try to at least purchase a cheepy one that is a brand you recognize. I also suggest trying to target no more than around 50-60% of RATED output. (It's a really good idea to double check that on cheap ones too). At least this way you have the illusion of some manner of warranty.
As a for instance, I purchased quite a few of the really poor quality refurbished EVGA 600W white units. They were $25 and I utilized them on systems with average draw in the 2-300's. I won't say they were trouble free in the long run, but they were cheap.
In that case you need an AT power supply ( not to be confused with ATX )
Now AT PSU's are very hard to come by, at least in good condition anyway.
There are plenty of ATX to AT cable harness cable kits. which , even when wired correctly can be a cause of great dissapointment.
Should you decide to take the conversion way, you should take a peek at this project: