Here are the part numbers you need from my 11-1957 parts book:
Sills, outer: LH 1493.26709, RH 1493.26712 "fianchetto pedana porta"
Sills, inner: LH 1488.10041, RH 1488.10042 "lungherone interno"
Rear valence: 1493.26706 "parte centrale posteriore"
Floors: 1493.10711 "pavimento"
Now, as far as I can see in the book, the valence corners are part of the quarter panels. This is a different construction from the 101 and the interim 750 Sprints I've owned, on which the trunk floor protrudes outside, the bottom of the fenders and top of the valence being spot welded onto it, forming a lip hidden by the bumper.
The floor is something else. This part number is for a single piece, from behind the firewall to the tail of the car! It is unlikely that this panel was ever used by anybody as a replacement...
My guess is that the floor stamping was supplied by Alfa to Bertone (or Pininfarina, or Zagato) and consequently was not changed when the 101 body was introduced. So any 101 replacement floor panels could be used.
If you have access to a talented panel beater, any reasonably affordable panel can be adapted and will be easier than starting from a flat piece of sheetmetal.
When I was trying to restore the 101 Sprint, I had a panel beater redo the sills, door bottoms and generic rust left and right. One key area was just below the headlamps: on both sides there are mouths for fresh air intake, which are rust traps. To get everything welded I paid 400 hours of work. I eventually bailed out of the project.
When I bought the car, two brand new panels were included: the two corner valences. Obviously hand made, these two panels had part numbers hand written on them by a milanese craftsman over 40 years ago.
You may also want to look at this Canadian source of panels:
Alfa Romeo 750 101 Roadster Body Panels
No Sprint parts are listed, but they were working on an interim 750 Sprint years ago and I sure they would adapt parts to your needs. Contact the owner, Allen Hall, for more info.