The easiest way is to take the part that holds the rubber insert to your local parts store, in most cases they will be able to get a replacement insert.

Tony

Neither the owner's manual or Resto-Pack specify the wiper blade size. It may be listed somewhere I don't have reference material for but the easiest way is to just measure your old one. Another way, if you're a club member, is to click the Rosters/Vehicle Roster button and search for 1948 owners. Their contact information will be listed. You can contact a fellow member and ask them to measure theirs for you.

I have a few 47's and 48 chevrolets, all of mine use 9" wiper blades.

Yes, 9 inch is what's on my '48.