B&H obviously. I'm always overwhelmed at the selection they have when I go there.
I've tried the brand "Inkpressions" which is inexpensive and they have a line of paper specially made for Ultrachrome inks. I know their matte paper is horrible with dye-based printers.
Lumijet is a pricey brand that
B&H carries, I believe it's supposed to be one of the most archival printer papers available. A great paper of theirs which I tried was the "Classic Velour" which is like a hotpress watercolor paper, but with an almost "cotton-like" fabric surface.
Of course the most expensive are the papers which come from the rustic fine-art paper mills i.e. Hahnemuhle, Arches, Legion...
You might also want to look at the "Pro-jet" papers which are the store brand papers of
ADORAMA (
http://www.adorama.com ). They seem to rival the quality of the more expensive brands. I've only tried "Smooth Silk", but I can tell you it's inexpensive stuff...the pros who come into the store seem to love the brand.
Speaking of the r1800...I WANT ONE!