Every page has at the top (between the banner image and the sightings/sales box) a bar that has a tab called "search" (third from the right).
This tab can either do a quick search of the entire sit, or you can click on "advanced search" and narrow down fields (like sections, users, words, dates, etc).
Also, as Demonac noted, you have the "search this forum" on the far right of the main table in each section.... and "search this thread" at the start of each thread when you go to read it.

If it is something else that I'm missing, please give more detail, or an example on another site.