I got mine from Napa in Pickering. They had to think about what the part was for a bit, but they finally directed me to the right aisle.
I have had mine on since last Nov. with no CEL.
It sounds like you need the new bushings on your rear swaybar. If the car is still under warranty go get them fixed by the dealer. If it is not under warranty but you just bought it from a dealer, go ask them to fix it anyway.
Also, if you search on 'clunk' you will find lots of reading...