Just for credit's sake, I want to let you all know that the bacgrounds that I put in my recent reference sheets are done with Kecky's brushes. Available for download here:

https://gumroad.com/l/NYgX

You can also see her tumblr:

http://kecky.tumblr.com/brushes

I also sometimes utilize CGtextures. It's free, and requires a little bit of credit in your work, and an account, bot other than that, it's a WONDERFUL resource.