Not sure on 2009 but on 2013 you can do it from the bottom by removing the plate/covers in there and bending the front piece of the wheel liner to gain access.
The issue is not getting to it, but unclipping it. You do not have good visibility and if you do not know how to do it is kind of hard. Using force will just break things - it's all plastics.
Sourcing it does not need to be from dealer but it has to be Toyota. I got mine from Estonia on eBay at much cheaper price that anywhere else. The fact that my truck has a less popular gray (there are many shades of gray Toyota is using) makes it more expensive than other colors.
You can lower the price even more, if you buy one that is for a very popular color (meaning a much more popular Toyota model than a 200) and then paint it yourself. In my experience that works fine with many OEM and aftermarket sensors, including Toyota/Lexus ones. The harder part is keeping the paint off the rubber gasket around the sensor (it is not removable).
The color is the letters at the end of the part number. Please note that they may vary for the same color depending on sensor location. For me is B6 in front but B3 and B2 in the back. You'll have to carefully research it if you buy on Inet based on part number instead of VIN.