Very simple really - anything that has the right thickness for the bullets I use, Usually I want the paper to add about.007 - .008 to the bullet.  Which is pretty thin indeed.  Preferably untreated paper, no coatings - but that's all.   
No need for 100 or 25% cotton or whatever, just solid enough to permit dry wrapping (in my case).

What I look for in patch paper is brittleness, or "crackliness."  This indicates to me that it is thinner than normal, strong enough to do its job of keeping lead from bore, and shreddable enough to come off the bullet as quickly as possible once out of the muzzle.  The more it disintegrates, the quicker it comes off, all else being equal.  This property doesn't seem to correlate with rag or cotton content, although I've found the general run of 0% rag, 25% recycled, computer printer paper to be absolutely unsuitable for patch paper.

I'm running out of my "classic" papers, such as Esleek Erasible Bond (last typewriter I saw was in an antique store) and K&E Graph Paper.  I'm pretty much standardizing now on Strathmore Tracing Paper, which is about 0.0015" thick, and Strathmore Vellum, about 0.0023" thick, for my "thin" and "thick" papers.

The Tracing replaces the Erasible Bond and the Graph Paper, being about the same thickness and consistency, and the Vellum is for making up windage using slightly smaller bullets.  I also have some NU Ultra paper, which is about the same consistency and thickness as the Vellum.

The Strathmore papers are available at Hobby Lobby and Wal-Mart.  I'd rather get funny looks from the staff as I apply my micrometer to the paper than try to figure out thickness from an Internet store description of "poundage."  To me, there doesn't seem to be any correlation between thickness and pounds per ream.

There is a very good dissertation called "Paper 101," on the Paper Patching section of the Cast Boolits Forum, if you want to read up on patch paper.

Roselle paper co  Tracing paper works well . Is about 0.00175"
No longer carried by Staples. 
Lately got several more pads of it on Amazon
beltfed/arnie

papermill fidelity 9# onion for .45-70 and .40-65

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