If this is for simple testing, troubleshooting, or any number of little DC mad science projects, I've found that a drawerful of second hand wall transformers is very handy. Many jobs can be handled with a 5A switch mode power supply from a laptop computer. They'll range from 12 to 20 volts DC generally. I've got half a dozen of these for a buck each from Goodwill. I've also got lots and lots of fifty cent standard wall transformers ranging from 1.5 volts up to 30. Most standard (non switch mode) wall transformers are both low current (usually in milliamps) compared to computer power supplies, and much less precise (and much cheaper to manufacture). If it's labelled 4.5 or 12 volts, for instance, a meter will reveal it to be off of that by quite a few volts in either direction. The cheap consumer items they mate these to just don't really care if they get EXACTLY 4.5 volts. An SMPS is usually spot on.
You can tell if it's an older style transformer, or an SMPS by weight. A similarly rated SMPS will weigh 1/4 that of a stacked plate xformer. A very few occasions arise that will not tolerate the signal conditioning that an SMPS puts out, and you must resort to the old-school transformer, but they are rare (diode circuits seem to not like them). SMPSs also don't like to respond to modern dimmer inputs (for similar reasons). A simple dimmer input feeding an standard AC/DC wall transformer is a cheap and handy way to peg the voltage output if you need it to be right on the numbers, or if you need a very light duty adjustable power supply (maybe you want a variable speed muffin fan placed somewhere).
For needs beyond 5A then a more dedicated bench-top supply, or a module out of a PC is needed. Brand new though, this stuff is pretty pricey.
The best quick and dirty, occasional use, high amperage testing tool is always going to be the humble benchtop car battery and trickle charger.
