I prefer notebooks to pads. My favourite is by a company called Phoenix Copper Art, that make notebooks with faded parchment paper, and make copper covers with commissioned engravings, bound with a leather string.
Whatever I find in an op-shop, which happen to be mostly notebooks. Other than that I recycle paper & shopping dockets that don't have writing on the backs of them! I now have a huge envelope of paper I can write on the back of!! I'm always looking for paper to write notes to remember something I've seen on line!