Any of the American Crafts 12"x12" page protectors for post-bound albums should fit. I've linked some for you.
The pocket protectors are great if you're planning on doing a Project Life-style album, or if you just want to display some of your photos on scrapbook pages, and others (maybe the less good ones!) all together. Hope this helps!
Some are designed for the 3-ring albums and some for the post-bound ones. I actually don't know what the difference is - maybe it's the distance between the rings, or maybe the strip where the rings go is thicker.
You will also find that other makes of page protectors will fit into your AC album, but I'm afraid I don't know which ones. If you ask in General Scrapbooking, you might get more replies.