Andi, the first link you provided is the Lee page for the color correction (warming/cooling) filters. You order those based the wratten filter designation (81x or 85x). An 81EF and an 85 would provide two reasonable powers. The Lee color compensating filters are here. The ones you want are probably the CC30M and CC50M. The "CC" refers to "color compensating", the "M" is for magenta (the appearance of the gel) and the number is the strength.
I don't know if B&H is an option for you but they sell Kodak, Lee, and Opti-Flex. The Kodak's are gels and are very thin. The Lee's are polyester and somewhat thicker. The Opti-Flex ones are too thick to use in combinations with your rear gel holder. You have to choose a size, but 3x3 is plenty of material. Here is the B&H site for these gels: http://www.bhphotovideo.com/bh4.sph/FrameW...SID=F6C998CE940