I started collecting South Park figurines when I saw them on clearance at Hot Topic. They are available online at MezcoToyz.com

Here are the ones I have:

Series 1:
Cartman
Big Gay Al
Officer Barbrady

Series 4:
A.W.E.S.O.M.E.–O.
Jimmy
Terrance & Phillip
Goth Stan

Others:
Deluxe Talking Mr. Hankey
Deluxe Talking Towelie
Boy Band Boxed Set
Boy Band Boxed Set (variant)

The following ones are available soon. Mezco is accepting preorders:

Series 5
Series 6
Deluxe Talking Jimmy