Is it possible to buy on Saturday?

The 8th generation iPad is available as of the 18th. Locate a store near your and schedule a shopping appointment. As far as I know, they take cash.

https://www.apple.com/retail/

I would ask to be sure that they have them in stock.

EDIT. I just checked stores near me. They are not in stock. They have to be ordered.

If the Apple Store is open (in many locations stores are closed because of Covid) and they actually have the model in stock you want to buy, yes they'll take cash.

We don't work for Apple on this user to user only forum. If you want to buy in-store, and it's open (check before you go), go early and take your chances. From what I'm seeing, people are ordering these in high quantities and many are seeing delivery dates weeks from now, so don't be too disappointed if the model you want isn't in stock when you want it.