You can buy everything but DIY is more fun
For your luck the SiRF star III
modules have become quite cheap these days. So you can use one of those with SPI or RS232 interface (have a look at your favourite semiconductor and electronics shop) as a base. All you need now is microcontroller and some memory. I would suggest an a SD card with FAT filesystem.
There are plenty of projects like this out the net. So it shouldn't be a problem to find hardware and software.
Otherwise you can adapt a project and just change it to what you need.
For transmitting the data there is an easy way. Since Android mobilephones (yeah I'm a fan of it) or iPhones are pretty cheap you can use them for sending the current position. This can be made per SMS (for long distances), WiFi or Bluetooth (for short distances). The second ones are able to livetrack your do so you can track him.
You also can use JUST the phone for complete work (GPS tracking, sending data, and so on).
However there is also another way. You can buy a GSM-module and pu it together with your microcontroller.
Last but no least, here is a project:http://www.opengpstracker.org/