Im assuming youre wanting something like this guy's shield
http://incunabulum.co.uk/blog/wp-content/uploads/2009/09/rmyr.jpg
Unless you find it on ebay, i dont think youre ever going to find
FW bitz for sale. Better off kitbashing it, since unless you want the particle effects its probably the easiest thing to kitbash for tau anyway. And dont do the particles
lol, im fairly decent at kitbashing and thats hard as hell even for me.