I bought a 2 pound beef liver 2 months ago and the day that I purchased it I cut it up into bit sized squares for the P's. I spread it out on a tray, put plastic wrap over it and whenever I decide to feed them beef heart I just go to the freezer and break of a piece. Defrost it and they are fed for the day. Easy, simple, and cheap.
get beefheart or beefliver, and make sure you cut the fat out before you freeze it. Good luck. Shred.