Dry Wine recommendations? Winery.

Long story short - not fancy, use whatever white wine you have on hand or like or have easy access to. Or do some shopping for fancier ingredients for a special occasion. Your choice.

Long story long - there's a wide range of choice for the liquor (main and accent) used in this soup. For main, white, red or fortified wine (sherry or port) or cider. For accent, brandy, cognac or Calvados.

Even though it's famous for being served in the morning at the now defunct Les Halles market), this dish is not haute cuisine with strict rules.

In fact, it has more in common with French provincial home cooking where people used every bit of what they had on hand (one of the original "nose to tail" cuisines).

I personally learned this dish both from my mother's home cooking and Julia Child's recipe (which uses unspecified white wine & cognac).
http://juliachildsrecipes.com/soup/julia-childs-french-onion-soup/

However, I have since learned variations in attitude and ingredients from:
1) Nigel Slater, for reminding us of relaxed home cooking (he uses unspecified white wine)
https://www.theguardian.com/lifeandstyle/2010/feb/21/french-onion-soup-recipe-slater

2) Serious Eats which gives an exhaustive and maybe exhausting treatment of which onions to use and how to cook them (they use dry sherry)
https://www.seriouseats.com/french-onion-soup-recipe

3) Felicity Cloake series in the Guardian, where she views and chooses ingredients and methods from several top recipes to come up with a "perfect" version of a famous dish (she uses cider and Calvados)
https://www.theguardian.com/lifeandstyle/wordofmouth/2012/mar/22/how-to-cook-perfect-french-onion-soup