Here is how you add a buyzone to your Counter Strike Source map.
Thanks to Darkone for this tutorial.
If you want a buyzone, do the following.
1. Create a box using the trigger texture where you want your buyzone to be.
2. Right click the box, and choose "Tie To Entity".
3. When the menu opens, on the left dropdown under the "Class" heading choose func_buyzone.
4. Once you have the func_buyzone added in the Class Dropdown, Look below it for the heading "Keyvalues"
5. Click on the "Team Number (int)" that is in the key values list.
6. Once you have Team Number (int) selected look on the right side
and you will see another dropdown menu with a "0" in it. Drop down the
menu, and pick what type of spawn you want it to be! Terrorist or
Counter Terrorist.