On the Philofaxy Facebook page, I asked if anyone knew where in the US to buy A5 paper. I did get some suggestions, but the best one (in my opinion) was to go to an office supply store, buy a ream of paper, and have them cut it to the A5 size. It was $5 more for them to cut it for me, but I was able to find the paper weight that I wanted, and now I have 1000 sheets of A5 paper.
My printer does have a setting for A5 paper, so I just popped it in, and printed this cute "Important Date" printable from Organise with Katie on Etsy.
And it's in my Filofax! Now to fill it in!