'03 Helm manuals seem to be rare. After I bought my car it took me almost a year to land one on Ebay. Keep looking, they do show up. I lost 3 because they went for such ridiculous prices before I got this one. Even then I think I had to go about $65 if I recall correctly.
I got 1990 manual new from rockauto for about $75. BISHKO reprint... yours being a 2003 you can probably still go to GM/Cadillac and order one. Never bought a FSM from GM but I have from Chrysler and they were $95 IIRC. I'd imagine GM would be similar?
..........or, if you don't mind a bit of laptop surfing, take a look at www.alldatadiy.com . The GM service manual plus a whole lot more - DTC's, TSB's, diagrams, labor rates, flow charts, specifications, torque settings - you name it.