Look at the Group Buy under Abay. I called up the Porter Cable warehouse here in Vancouver (actually Burnaby) to try to get a counterweight and the backing plate, and they said they couldn't sell me the counterweight without the adhesive backing plate as a kit, for the sander ($30!)
99ESMAX knows of the places to get the correct counterweight, and the pad prices are better than any place you will find locally. Trust me, I have looked EVERYWHERE!