I think here are many different page protectors that will fit.
I use C-Line page protectors in my D-ring albums.
I did buy a different brand of page protector in the past and they were too wide.
(Hopefully I can explain this... )
I found if pages have an extra space/column along the holes, they tend to be too wide and stick out the edge of the album.
I linked two here that should illustrate what I'm talking about. The post bound protectors will fit (the holes line up, etc.), but because of the extra space near the holes, they are too wide.