Cheaper cameras don't include external power supplies in their boxes. In mosts cases, the power supply is unnecessary, because one set of batteries often lasts for an entire day of shooting -- or longer.
If you do want to buy an external power supply, it will be expensive compared with power supplies in general. The CA-PS700 is the model for my Canon PowerShot S1 iS. This Web page lists it in the range of US$43 - $85. In Canada, Future Shop lists Canon power supplies in the range of CDN$70 to $300 -- depending on the model and capabilities.
(A curiousity: the S1 iS uses 4 NiMH batteries which output about 4.8 volts; the Canon-supplied power brick provides the camera with 7.4 volts.)
That gets some people wondering about using a third-party power brick. Over that the dpreview forum, technicsplayer made these comments:
* The socket is centre positive.
* Portable digital regulated supplies with a 7.5V setting work well with the S1.
* It also works with 6V.
* Google for "power runner" is a smallish Li-ion pack with convenient belt clip. The top model is rated at 5Ahr at a regulated output of 7.5V or 6Ahr at an output of 6V,
Be careful though: cheap unregulated supplies may damage the camera.