Hello sassypineapples. You'd want Sodium Nitrite, not Nitrate if you're looking into this method.
'SN: The main compound for this method, Sodium Nitrite is easy enough to find. You're looking for >98% purity Nitrite. This chemical is sold without regulation and can be bought from Amazon, eBay or any lab supplier in your area. 100 grams cost between 8 and 10€. This chemical is completely legal to own and is used during curation of meats to preserve their colour. It's described as "White to yellowish powder/crystals" and it's said to have "slightly salty taste". It is also very soluble in water.
The recommended dose from the PPH is 15 grams, however, this has increased on different issues, from only 5, to 12 and now to 15.
Very important: You're looking for Sodium Nitrite, not Sodium Nitrate; NaNO2, not NaNO3.
Should you not find it just by looking up Sodium Nitrite, look for: NaNO2, NNaO2 or Filmerine. Make sure you're buying what you want and that it's purity is high enough. For more information on it plus some industrial sellers, check Sodium Nitrite on Pubchem.'
Edit: This is from the first page of the SN Megathread that Nofaith linked (I know it's a lot to go through). There's a chapter in the August 2018 Peaceful Pill Handbook on lethal inorganic salts that you might want to look at. You can find it in the resources thread.