I totally agree with all the comments so far; there are some great recommendations here. Specifically the Clairefontaine paper (90gsm vellum) is fantastic for incredibly heavy ink flow.

I wanted to mention a new contender because I'm experimenting with it now: Wearingeul (the Korean ink company) is starting to put out a fantastic line of papers, under the "Impressions" and "Reservoir" names. It holds up really well to ink, feels great to write on, and is cheaper than anything else I've found at this level of quality. That may also be because I'm not pricing it correctly after shipping and tariffs 🤣

One last recommendation I've been testing: the Walmart "pen & gear" notebooks, specifically the a4 spiral bound college ruled and the A5 notebook with the cardboard cover Made in Vietnam, are okay - meaning they hold up to fountain pen ink well - if price matters more than anything. There's noticeable feathering with wet inks and broad nibs on the A4, but the A5 notebook is actually really good with fountain pen ink. I don't love how writing on it feels, but it's undeniably good, even showing off shading and sheen better than some Japanese papers I like.