All the codex pages should be vissuable on your maps, check al your maps again, it must be there somewhere.

Try buying all the treasure maps in each area.

Get every single Viewpoint you can in each region and uncheck all map markers except Codex Pages, in that order. Shouldn't be too hard to find.

You could do all the things the ppl say or you can continue with the story until it does not let you continue unless you have all the codex pages, this will put all of the ones you dont have on the map.

The codex pages show up on your map when you SYNCRONIZE VIEWPOINTS also, you receive a codex page after a certain mission, this codex page grants you the useless gun.

You have to get to memory sequence 14 to see where the last codex page is.

Near the end of the game you get a map which shows all the codex pages locations

go to assassinscreed.wikia.com/wiki/Codex and it will tell you the location of every codex page. i had the exact same problem as you as i was missing page 19 at the end and it was in the south east of Venice, somewhere i hadn't been before. this website is a big help.