Great suggestions guys!! And L4H, thanks for the vote of confidence.
My thought on this, and you may not agree, is to be patient. You said she let you pet her before she had the kittens. Let her keep coming to you for food. Pet her every chance you get. Keep an eye on what direction she goes when she leaves but be aware that that is not necessarily where the kittens are. She may head off in a different direction to throw you off. If she feels threatened in any way she will move the kittens, regularily. So even if you found them she will move them, again and again.
My point, and I do have one
, is to get as friendly as you can with her and be very very patient. If she feels comfortable with you she will bring the kittens to you, just like badger says.
Is it possible she belongs to someone? Does she look like she has been on her own for a while?
Good luck to you kitty