I found a brush guard from a 2001 Disco for sale. My question, do they make an adapter to fit to a '03 Disco?

 

I've not heard of one. The guard you're looking at requires 1 or 2 holes to be cut in the bumper for the bar to pass through. There is a template included with the mounting instructions, but it probably won't line up to the newer bumper on the '03-'04.

 

Right, 99-02 DII's have different mounting points than 03-04 DII's. I have never seen an adaptor set and I dont think it would work because the front bumper on 03-04 is different than a 99-02: