As a hobby bookbinder I’ve always found difficult to find material to bind a ‘real’ book. Here are some tips:
Inkjet/laser printer papers
The easiest thing you can do is to buy some larger (A3) printer paper and fold it to size. For this, it would be great to know the grain direction. Unfortunately, that information is rarely available (except in specialized bookbinding shops).
It’s better to start with an even bigger size: A2. That way, you have much more possibilities for folding and keeping the proper grain direction - regardless of the paper’s original grain direction.
For a start, I can recommend an A2 Schoellershammer Reflex marker paper 75 g/m². That size is big enough to create A4/A5 folded sheets economically.
If you are not a company, buying printing paper in full-size sheets could be a bit tricky, especially if you are not ready to buy a ton. Luckily some paper factories offer ‘sample’ services. I could only recommend Gmund Paper. Their collection is excellent. You can get samples in different sizes, from A4 to full size (1020x700mm). In addition, they provide information about grain direction. I like all their papers, but especially the Lakepaper Extra series.
If you don’t want to fold the paper by yourself or can’t find a proper paper, there’s also a chance to buy some pre-folded sheets.
You can also find pre-folded sheets. For example, in Germany, you can search for ‘gefalzt Kanzleipapier’. Office supply stores sell A4-sized empty/lined papers (A3 sheets folded in the middle) in bulk. Those papers usually have 70-80 g/m² paper-weight and lower quality.
If you are looking for something high-end, you can check the ‘Star Notenschreibpapiere’. This company sells blank music sheets in all different sizes and formats. Of course, you can buy folded sheets too.
Publishers who sell books in sheets for binding
If you want to bind a ‘real’ book, here are the (super short) list of publishers who sell books in sheets for binding. Please contact me if you know any other publishers, and I’ll update this list.
Legacy Press is publishing works about printing, paper, and bookbinding arts. They sell some copies of their new books in sheets. I bought the European Hand Papermaking by Timothy D. Barret back in 2018.
Fines Mundi Verlag
They sell bookbinding related, facsimile reprints (out of copyright), printed on good quality paper, sold as sheets. The reproduction and the paper look pretty good. Search for ‘Rohbögen’ on their page.