Since this is books-about-paper month at HM, thought we'd post links to a few sources of handmade paper...
Barcham Green stopped making paper in the mid-1980s, but Simon Green has keeping the firm's history alive, and is also selling off what stock remained. Here's a list of what's available and the prices. Simon also collaborated with Claire Van Vliet's Janus Press to publish Papermaking at Hayle Mill 1808-1987, which is an excellent history of the firm.
Whimsie Studio is a Florida-based site that offers a wide range of arts and crafts-related materials, including a selection of vintage English handmade papers. They also have some interesting print- and paper-related books on offer (like this one).
Ireland's Griffen Mill continues to make a wide range of papers; ones suitable for printing can be seen here.
The University of Iowa's Center for the Book is making some beautiful Western & Japanese handmade papers, under Timothy Barrett's direction. HM's latest, Paper Should Not... includes a sample of their Chancery paper. They don't have lots of stock lying around at any given time, but they can make batches to order, and at very reasonable prices for what they're producing.
Atlantic Papers carries Vekle Losiny, but it's hard to tell from their site if they carry anything suitable for (book) printing; seems to mostly be stationery.
Twinrocker continues making lovely papers. It is, however, the one contemporary mill not included in our Paper Should Not..., but that's only because none of us had a stash of their paper small enough to contribute to the project.
Montreal's St Armand can make paper to a wide range of specifications. They don't generally have stacks of white book-weight lying around for a project, mainly because people ordering that kind of quantity want the paper customized to their needs and preferences.
And of course, HM's own favorite, Reg Lissel, can be contacted through this page. Think he's again made paste papers for the special copies of the next issue of Parenthesis...
This list is far from exhaustive. However, while papermaking remains a lively craft, few people seem willing & able to make a simple white sheet, consistent & in sufficient quantity for printing a book.