There are ways to get hackable brand new PS3's.
First of all, look for 120 or 250GB slims, they will have FW 3.30 or 3.40 at most.
As for the 160/320gb, your best bet is getting a 320gb bundle that contains a game with FW 3.50 or lower. For example the GT5 bundle, or maybe the COD black ops bundle?
Avoid the 160/320gb without game as you cannot know which FW it will be and it'll probably be 3.60+ (which means unhackable, since it cannot be downgraded).
PS3 Slim CECH-3004A 160GB
PS2 Slim SCPH-70004 - FMCB 1.8b