I am pretty sure you can use any palm/ pocket pc games for your zire 72. The all have the same cartridge, this is to make it more convienient for everyone. You can buy the games anywhere, best buy, circuit city, target,...etc.
Oh, but since you said free, try http://download.com thats where I have mostly downloaded my games for my zire 71.
Well...everybody loves Bejewled. So go ahead and trey that game out. Its very addictiing. Also, here...http://www.palmtown.com/ go there. They have a a few more games.