If you do any amount of writing in your Filofax (and the A5 size is just the right size to do a lot of writing) you'll be shocked by the cost of plain and lined refills. I'm not the first person to suggest this, but it really pays to make your own.
I dug around in the stationery cupboard and found one of these:
This paper is very cheap, but of surprisingly good quality. It's particularly good for fountain pens. If you can't find one of these, look for a pad or wire-bound book that is A5 and has perforated sheets. These ones come away very cleanly:
Be sure to check that the paper is A5-sized AFTER you remove it from the pad. Otherwise, you'll end up with paper that's slightly narrower or shorter than A5. This one is perfect and after punching fits straight into the A5 Filofax:
This is a great way to have paper significantly better than the Filofax packs at a fraction of the price. Buy several pads if you see them on sale.