Rick’s suggestion of a car battery on a charger is a pretty good one.
If you are going to use a bench type power supply, there are two basic types.
A linear power supply will have very little electrical noise and will properly regulate itself over its entire load range, so small loads or no load isn’t a problem. You don’t see many linear power supplies these days, because they are physically large and generate a lot of waste heat.
Switching power supplies will have a lot more electrical noise. They also usually require a minimal load in order to regulate themselves properly, typically somewhere between 5 and 10 percent of their maximum rated load. Switchers are smaller and more efficient than linear power supplies, and generate a lot less waste heat.
One trick to use a switching power supply with a very small load is to load the power supply down with something else, like a powerful light that also works as a convenient bench light.
A car battery can supply a heck of a lot of current. Bench supplies that can crank out a lot of current get very expensive very quickly. For most things you don’t need to supply anywhere near the total current capacity of a car battery (unless you are testing starter motors for some reason) but if you are testing car radios, some of them can draw quite a bit of current, especially those with separate amplifiers and subwoofers and huge speaker systems and all of that. If you are testing this kind of high power stuff, the car battery on a charger may be by far your cheapest and simplest solution. The car battery on a charger solution can also handle a very wide current range, much wider than any bench type power supply that you can likely afford to buy.
Computer power supplies are almost always switching supplies. Turning one into a bench supply is quick, cheap, and easy, but does generally require putting some kind of load on the supply so that it will regulate itself properly. The 5 volt on a computer power supply is where all the beef is. The 12 volts is relatively weak. Computer power supplies are a much more attractive source for a bench supply if you are tinkering with 5 volt electronics.
The voltage on a lead acid battery is about 11.8 volts when it’s pretty much dead to 12.6 volts when fully charged. In a running car, the alternator will boost the voltage up by a volt or two, so anywhere from about 13 to 14 volts or so is pretty common, maybe up to 15 or 16 volts if you rev the engine (depending on the car). A quick and dirty test to see if the alternator is working is to measure the voltage when the car is off (you’ll just read the battery voltage) then measure it again with the car running and make sure it went up by a volt or two. A car’s electrical system is also very noisy. Not only does the alternator create a fair amount of ripple, but the spark plugs can generate a rather large amount of wide frequency noise. Keep these things in mind when you are testing things on your bench supply. The actual electrical environment of a car is a bit more harsh than that of your typical test bench.
