I don't mail order hardware like this. A good hardware store will have or be able to order.
From my 3B Parts Manual (PM) and I'm thinking it should be the same:
Screw, Ov. hd. No. 10 x 1/2".........12
Washer, finish, No. 10 .........12
I don't know where you're located but, when in KS, I drove over to a huge store in KS City to get just two correct step bolts for the rear deck when so many folks were pushing the correct bolt was an elevator bolt. I did convert to phillips and agree the sloted head will be the challenge.
Since I knew I wanted all seats, I purchased enough to get all the seats done (yeah, they set in a bag for a very long time until I could get to the covers).
