Thanks, that gave some clarification.
However, concerning the suggestion to choose according to my design: That's exactly my problem 
If I don't know the possibilities, I cannot choose the right one. Unfortunately, there seem to be a lot of possibilities around. A table with the most common components would be neat. Until that, is it a good idea to just search "switching regulator 3.3v" on ebay and then check for the input voltage range and maximal load? (Ebay, because that is the only supply I know in Germany, where shipping is not 5 times higher than the component price.)
Regarding designs, I think there are not to many possibilities, at least for home use. There are 3.3V, 5V, and 3.3V/5V setups, and there is power from the wall and from a battery pack of AAA/AA batteries. (I would choose 9V block, as they are expensive, have less capacity and the rechargeables are not as good).