If your looking for the actual blank textures, go here:
http://simviation.com/fs_paintkits1.htmotherwise, as mentioned above, use dxtbmp, OR my prefered program: Imagetool, to convert the textures into a format you can edit (either 24 or 32bit) and then back into a format FS will read (commonly 8bit (aka 256 colour) or 32Bit, or dxt3)
(I can get download links to both if you need)