Sailor sells empty cartridges for syringe filling. While this sounds completely pointless given you could just buy one filled with ink and then use the empty I believe they are made with more robust plastic that prevents stretching at the neck after several uses and the potential for an unexpected leak or excess flow at the nib. Something to look into.

Midori MD pads with one of their covers work well and aren’t too expensive. Available in A5 and A6 if I recall. Otherwise as already mentioned I like the 90gsm Rhodia or Clairefontaine notebooks. The 80gsm Rhodia paper (found in a lot of their pads) always feels like the backside of the paper has less coating on it and not as enjoyable.