Gibson absolutely will not sell their Y-Pipe apart from their cat-back systems......believe me, I tried. I didn't check to see if Banks would sell just their y-pipe. I'm guessing not, though.
I had my local shop custom fab one for me while installing my new Aero-Turbine muff & reso a few days ago. They ran (2) 2.5" into single 3". Mandrel bent would be better, but for that small amount of pipe, I don't think you'd see much difference, if any. Cost was around $150.
Steve