You can use any job searching site and subscribe to vacancies which contain words "mathematics" "math" of whatever.
In Russia I am subscribed on www.hh.ru - I am getting about 10-20 vacancies everyday, it is for free, absolutely most of them are not really related to math, but sometimes something related appears. But I guess in USA you will get much more vacancies where mathematicians are required or at least welcome. I think http://www.monster.com/ is Okay for USA.
I am sorry if this rather trivial answer already has been posted - I looked answers and did not see one. Any way I think your question is very good.