Two solutions present:
for small quantities, a strong smooth metal straight edge and a pocket craft knife - the sort that where you can break off the blunt edges and get a new sharp edge. They're the sort that will get you lots of attention if you try to board an aircraft with them. That's how I cut my A5 Photo paper and my notelets.
For large quantities, is easier!
Buy your paper from your favourite stationers take it across the road to the photocopy shop that has a guillotine machine and ask them to cut it to size. They can do this extremely accurately and consistently and will charge you by the cut, not by the number of sheets. I had 500 sheets of mixed colour Filofax size paper cut and the cost was a fraction of the Filofax stock paper.
Not all Photocopy shops have guillotines.