Hi!
you can order it at meilleur du chef, they have the louis francois brand which I use in the restaurant. https://www.meilleurduchef.com/fr/achat/patisserie/ingredients/conservateur/lfr-citrate-trisodique.html
For cheese fondues it's usually white wine warmed to scalding with either some acid (vinegar, lemon juice, gastric, verjus, whatever) or some alcohol (cognac or something 40% ish) and then whisk in the cheeses bit by bit until emulsified. If it's 'wet' cheese most add a little starch. Not really my cuisine though so maybe there are other tricks.