Depends what size you require & how much you're prepared to pay.
Agfa Gevaert def make APX 100 in 6.5 x 9 cm.
You can get Slavich plates in a variety of sizes.
I've also been told that Kodak do special runs for astronomical photography & other types of scientific testing.
Last I heard you are looking at approx $13 per plate from Slavich & Agfa (that's Aussie dollars).
In lieu of a trip to Denmark (although you wouldn't believe how tempted I am to buy a ticket just to see Gandolfi's work!) here is a doable starting point: