It's hard to find a really good brick/mortar stationary shop anymore, but there are a number of excellent suppliers on line.
Shop all paper here, including notebooks and notepads, planners, stationery sets, loose sheets, index cards, envelopes, and more.
www.gouletpens.com
Not all paper is created equal! These notebooks, notepads, and memo books use high quality paper that is perfect for gel, roller ball, and fountain pens.
www.jetpens.com
Everything you need for all your paper needs - journals, notebooks, stationery, calendars and more.
www.dromgooles.com
Also, the major arts supply store chains such as Michael's, Dick Blick, JoAnn, CheapJoe's, and their regional counterparts often have a good selection to choose from. Most towns have at least one locally-owned art supplies store that maintains a small stock of writing paper.
One approach to the problem might be to search for an identify a brand of paper, and then shop around for retailers who sell that brand. Rhodia and Clairfontain are both highly respected European manufacturers of fine writing paper. In the niche world of fountain pen and ink testing, Rhodia is pretty much considered the standard paper for comparative tests. In the US, Cranes used to be the gold standard, but no so much anymore - they are still the sole-supplier of paper for US currency (and the only company to ever supply this specialized niche market), but they have all but abandoned the ordinary writing paper market. In fact, there was a news story about a month ago announcing that they were closing their facility in North Adams, MA and merging with Mohawk Papers in Cohoes, NY. Interestingly, the main focus of the company today is plumbing fixtures and accessories. Go figure!