I don't know if you can get them anywhere other than ordering direct from Porterfield. Call their 800 number and tell them what you want. I've always found them extremetly easy to deal with (I'd like to meet the girl who's taken my orders over the years.)
I've actually not used any other pads at the track than Porterfields (and a brief stint with PF street pads), so I don't have a basis for comparison. I can say, though, that I've not had any complaints about the Porterfields. They're a bit pricey, they can be ******* rotors, but they're really great on track.