I make my own envelopes out of high quality, archival grade, bleach free (etc) paper. An A4 sheet cut into 2x A5s with a few folds and 2 staples or Sellotape to secure the ends works fine. A ream of this paper is not expensive (per sheet) and will last a lifetime. I do the same, but larger, for 35mm & 120 negs.

Admittedly these are then just stored in a small 'shoebox' with a bag of silica gel (changed regularly once every ten years

).

I guess the d-i-y envelopes could be slid into a 35mm album style system if necessary .....