What you want for free motion quilting is a darning foot. I would expect that the Brother site lists a darning foot; you could look at that.
A generic darning foot will fit your machine, but it has to have the right shank. Here is a website that tells you how to determine the shank on your machine:
http://www.clotilde.com/pages/which_foot.html
If you go to JoAnn's, you can bring one of your brother feet along with you to check the shank length of the new foot against the shank length of a foot you know works on your machine.