Babies R Us in Union Square, buy buy baby. And I don't know if they would be under 400$ but Schneider's Baby & Teen Furniture on 25th has some interesting cribs I hadn't see at other places. They're probably pretty expensive though. Albee that PP mentioned has some good deals online so I bet they'd be good to visit although I haven't been there in person. I went to buybuy to see my crib & then ordered it on amazon.com.
Re: Where can I go to actually see cribs?
I would say
Albee Baby at 715 Amsterdam Ave at the southeast corner of 95th Street
or Buy Buy Baby at 270 Seventh Ave. (Between 25th and 26th)