If this product could be easily available and cheap, it shouldn't be necessary to prepare it from sulfurous acid like shown in the post before this one. Searching the internet and knowing what this substance is used for, I found it at the site of Aqua Ambiente, with the name of code CTX-12 and known as Chlorine neutralizer for swimming-pools. It is, in fact, Na2SO3, CAS N. 7757-83-7. Besides, they sell the same product with the code name CTX-14 as Bromine neutralizer for swimming-pools.
I already asked for the price of a 6 kg package, I don't think it will be so expensive and can serve for the preparation of developers and fixers.
But I found, by accident (almost) another way to prepare a fixer.
Yesterday I bought very cheap 5 kg Ammonium Sulfate package at an agriculture drugstore. What is this substance good for? Well, I knew it would react with sodium hydroxide, giving something else. So, I prepared two weak solutions of each one and then poured base over sulfate. I made this at my balcony in fresh air, to avoid explosions inside. No explosion, but I could smell ammonia in the air beeing produced in that reaction.
In fact, searching the internet, I found out:
2NaOH + (NH4)2SO4 ---> Na2SO4 + 2NH3 + 2H2O
Ammonia NH3 is also a fixer. And now I will start a new way to prepare another fixer... This time based on ammonia instead of salt.
Actualization: Ammonia is not a good fixer because it doesn't dissolve all silver halides (see in the page Recipes the final note)