I don't believe the PPEH provides any specific sources for SN; instead, it recommends using their members-only Peaceful Pill Forums for the most current information. As far as I recall, there weren't any listings in the PPEH detailing where to purchase SN.
Also, see this from the PPEH:
View attachment 152355
This observation is based on the 2022 edition. The 2024 changes larged focused primarily to Nitrogen gas and some sources for gas, and I don't think it includes any updated information on SN. If it does, I welcome anyone to clarify that! Given the 2022 discussion regarding the fluctuating availability re SN, it seems unlikely that they would include such listings in the 2024 update, especially considering the increased and increasing restrictions of it in a lot of places.
Hope you find evertyhing you are looking for and get peace & serenity.