Quite a response to the post about ink, both in comments and in email. The obvious next choice is paper but if anything I know even less about that than I do about ink.
I use Basildon Bond A5 paper for writing samples and my notebooks are Tiger A5 Plain Spiralback. I get through many of them. Most of the other papers I use for correspondence are vintage from eBay. Old papers are generally kind to fountain pens, being intended for their use. I have a set of old Airmail paper and envelopes, no longer necessary as weight restrictions are not as stingy as they once were, but nice paper which I use for overseas correspondents.
Then there is a very old writing set I picked up, maybe 1920s or 30s, woven paper which allows no see-through despite being thin, with small envelopes which do not correspond to any modern size. I love that and use it often.
I do have some modern paper, Clairefontaine for instance, very nice but nothing special, really.
Truth be told, my interest in ink or paper is slight and peripheral to my fascination with pens. You need something to write with and something to write on, in order to enjoy your pens. It’s nice to have a variety of those materials but my money is spent on pens, not to any great extent on ink and paper.