So if you don't want to use '57 Chevy pans, where can you get proper Pontiac floorpans?  

MMMMMMMMMMM! I would have to say! On that year of Pontiac. You would have to find them at your local sheetmetal supply store or welding shop.
Well no kidding. If you are really good and love to fab. up stuff.
Go and get some construction paper. And start laying and cutting it. So it lays over what is left of your original floor. If you have to amke it out of serveral pieces. Tape them all together until you have the floor.
Then remove it and untape all of it. And trace them out onto the new metal.
I done a 1958 Impala a year ago. I had to make all of the rear floor. That is under the rear seat. They make the main floor panels but not that section.
DO NOT cut out any of the original floor until you have made the new panel to replace it with.
I tack welded the new panels in over the original floor. And hammer and dollied them into shape using the old floor as support.
OH! And you can also use some heat to help you bend and shape the new metal.
THIS TAKES LOTS OF TIME!
But once you have all shaped. Take the new panel out and grid down all the welded pieces. Then trace it and cut out the old one.
GOODLUCK!