1- If you want a standardised type of buche : chez Paul or equivalent
2- any local bakery or even supermarket might do fine as well, but insist on"creme au beurre' (butter sort of cream) as a topping if you like it. The rest of them are coated with vegetable fat (yark, to be avoided at all costs). Alternatives are chocolate powder coating or other coatings.
3- if you need the expensive type: go place de la Madeleine (Hediard, Fauchon, Dalloyau)
They are much more sophisticated and refined desserts (somehow more tasty) than buche which consist in a rolled genoise, filled with jam or fruit marmelade, and coated with Creme au beurre. I find it rather heavy (especially after a delicate meal) and would instead go for a Vacherin (meringue & icecream dessert, but not the cheese, with the same name) or any other dessert. A well prepared tasty Opera cake or equivalent would do fine. As it is customary to drink Champagne with dessert on Xmas, go for the Vacherin for instance (you can even choose the ice cream flavor) and I would humbly suggest Champagne Demi-sec (light sweet) instead of Brut (which has now gained popularity, but that I would reserve for toasts, or diner long wine) Metropole Paris - How the French Do Nol
The above can be endlessly disputed. Only your taste matters.
Try before xmas.
Depending on your tastes and your budget, I'd just make a tour of the bakeries in your area and start sampling their wares. The buches de noel usually start appearing in early December - and some bakeries offer little single-serving buches, in various flavors (chocolate, Grand Marnier, nougat, etc.) so you can sample before making up your mind for the big buche.