If you have Photoshop, there are two filters which can create normalmaps-- one in the Filterforge database (this program is commercial, though), and a free filter by Nvidia in their "Nvidia Tools" section or something like that.
I think there's a GIMP filter as well, but I've never used it as of yet.
As a general rule, though, when you use a Photoshop or Gimp filter to convert an image to Normalmap, you want to add a Gaussian blur of... what was it, 0.5 to 1.0 or 1.0 to 1.5? I can't remember. Anyway, you may want to up the contrast of the image after grayscaling it, then add the blur, before you use the Normalmap filter.
Edit: Sorry, I misunderstood the TC's post. Though maybe this info will be useful to someone...
Edited by IceEmpress on 05/29/13 12:16 AM.