Plenty of Note 10 and Note 10 Pro cases on Amazon

Buy one and have it available when your phone arrives.

I usually wait for specials. I get an expensive one for a $1 USD or $2 and then wait to see what reviews are like for other cases. These specials come before the preorder even starts. A simple case is better then none (for me).
 
And Ebay has joined the party! I guess Slickdeals will be next. They usually come through with good discount codes for reputable brands on Amazon.