Beemer Boneyard is also a good source for pads.
Also, if you are new to this job, be advised that the Clymer's manual's instructions for pushing the pistons back to make room for the new (thicker) pads make no sense at all.
What I figured out to do is: (1) Remove the fluid reservoir cover. Leave the cover loosely on top of the reservoir, to keep anything from contaminating the fluid. (2) Pull one of the old pads half way out. (3) Grasp the top of the top of the pad (actually the metal backing for the pad proper), and gently but firmly rock it from left to right, so that the bottom edge of the pad can push the piston in. (4) Repeat for the other side.
You may or may not feel the piston move. In any case you'll know you've done the trick if the new pad slides into place freely. If it doesn't, repeat the above.