How about DIY?
You can use either 78L05 (100mAmp), or 7805 (1Amp), or LT1804CT-5/LM1084IT-5 (5Amp). With any of those chip and couple caps, you can make your own 5V regulator for less than 5 bucks.
Search for the datasheet on how to make one.
oh yeah, if you are not DIY, Radioshack should have them. If you need one that doesn't require much juice, get one of those voltage selectable power adapter used for portable CD player that most place sell.