There is no such thing as an FHA 203k certified contractor. FHA does not endorse nor certify contractors. FHA allows the mortgage lender to "approve" contractors that are "acceptable" to the lender. So, you need to contact your lender for this info.
You can always use any contractor that your lender "approves" and there's always Home Depot ( http://www.homedepot.com/webapp/wcs/stores/servlet/ContentVi ) and Lowes.
There is, however, a Web site that may help, but not all areas have contractors listed yet.