I bought the Claessen's Double Primed in a roll from Jerry's Artarama at
http://www.jerrysartarama.com/
I got the large wooden arm palette from Dick Blick at
http://www.dickblick.com/ (If you paint with your right hand, order the one that sits on your left arm. The way they describe which palette is which can be kind of confusing.)
I'd like to hear what people who use the Claessen's feel are the advantages of the Single Primed vs the Double Primed or vice versa.