I use
Scrapbooks Please
Their Standard Matte 12 x 12 prints are usually $1.99, but every once in a while they have a .99 sale...and they have one going on right now until July 15th...
This company offers different qualities and thicknesses, and these are great for just going in a book. I have ordered from them many times. I just wait until they have a sale and then order like 25 layouts or so at a time. I have never been disappointed in their service either. On your first order they send you a little sample pack of what each of their papers are like so you can see which type you like best.