The link didn't show any power supplies but I thnk I know the gadgets you mean. Usually, they have a sliding switch marked 3-4.5-6-9-12 or something like that.
You can use these but normally they are unregulated. This means the voltage isn't actually stabilized to what it says on the switch, it is usually higher but drops to the quoted voltage under a particular load. If the load increases,the voltage continues to drop so if you plan to experiment with ICs. particularly digital ones with their fussy voltage demands, it probably wouldn't work. A regulated supply is one that holds the voltage steady from no load up to the maximum it is rated at.
Suggestion: If you get one of these unregulated supplies and add a simple voltage regulator circuit after it, it should do what you want. I suggest you look at the data sheet for the LM317 IC, with just a few inexpensive components you can make your own regulator to go after the unregulated one.
Brian.