Check out the Ka-Bar TDI Law Enforcement Last Ditch Knife,(try saying that ten times fast ), it's not actually a push dagger but it's similar and it is great!! I love mine!! http://www.kabar.com/knives/detail/79
As others have said be sure to check you state and local laws. In many if not most places *carrying* a push blade and/or dagger is illegal.
That said I think the best readily available choice is the Cold Steel Safe Keeper II. It has a nice hand filling grip, good size blade and enough belly to the edge to be able to slash as well as stab. I've got one as well as its larger OOP brother and think they are well designed and well made.
Cold Steel makes some smaller models, but I would not pick anything smaller than the SK II as a primary defense weapon unless I had to. I know TOPS and Benchmade also market similar knives but personally I still prefer the CS offering.
Most of the other production push daggers I have seen were cheap novelty items and not serious weapons.