Yah, your best bet is to try and find one at a junk yard or a cheap one online and have it repainted. Screaming Yellow isn't a common junk yard color lol. I don't think Prestige Mustang has any yellow 99-04 mustangs. You usually can get reproduction bumpers for under $200, I think Latemodel Resto has them, but "mustang" isn't stamped into the bumper.
I paid $600 for my GT bumper and to have it painted. I didn't really bother looking for a takeoff or anything. $600 does seem high, but the paint is an exact match and looks great. Plus they do full prep and bake. I'd expect to pay anywhere from $100 to $350 to repaint, but you get what you pay for...