If you have low power (low amperage) requirements you can usually scrap a power supply out of an old satellite unit or an old DVD player. There are plenty of other sources as well.
You need to tell us what voltage you want to run something at.
You need to tell us what amperage the "something" requires.
You need to tell us what your skill level is.
Is there a physical size limit to your project?
This is why we ask for more information.
Once, at an electronics store I worked at during summer vacation from school, a cop came in and asked for "A thing you plug into the back of one thing and plug it into another thing." He said he didn't know what it was called. I told him I don't know what it's called either, but I know what he wanted and that we didn't carry them. Told him to go to our competitor. He could have been more vague, but only VERY slightly more so.