Had to look up "A5" but it turns out that has been my favorite size for about 20 years. I didn't know you could get regular notebooks and refill paper. I've always used Mead or similar, which are sometimes hard to find. Lately I found that Daiso has them for $1-$1.50, so I buy a bunch.
I have stacks of them, they're great for seminars and when I fill one, I just label the cover with what's in it. I also use them as a daily log in my business, in coordination with a weekly planner that's the same size.
Edit- Just realized you asked about a journal. My journal is that size as well. I got it from Barnes and Noble, but I can't find any other brand name. It has a beautiful leather cover and says "Handcrafted in Italy". Hope that helps.