Where to find fancy flour in Paris?
It's not Retrodor brand, but any grocery store will be able to supply you with T55 -- it's the standard grade of flour sold in France, in 1-kg bags
T65 (again, not the specific brand) is also sold in 1-kg bags in *nearly* any supermarket -- It's almost always Bio (organic) -- but it's nearly identical to American all-purpose flour.
I echo the suggestion to buy it from King Arthur -- easier and cheaper than having your friend sacrifice 10% or more of her luggage allowance.
These are professional flours, sold in bulk. You have to be acquainted to the boulangerie milieu to purchase those, and since you usually get them in 20 kg bags, I'm afraid airline check-in is out of the question. Unless you can use the means of an export company, in which case I suppose getting the flours would perhaps be less of an issue.
It might be possible to purchase small quantities of any of those flours from boulangers, but I have never tried to do that.
Your first stop should be G. Detou in the rue Tiquetonne (métro Étienne Marcel). They supply bakers and pastry chefs, as well as the general public. Ptitpois is still correct, however, that most will be in bulk. It might be smaller than 20kg sometimes, but G. Detou does do some repackaging of bulk products. I just don't know about flour.
Your second stop should be the website http://www.kingarthurflour.com. They make excellent artisan flours. Their professional catalog is filled with detailed technical info. The Sir Galahad is a very close equivalent to a good quality T55 French flour. It's sold in bulk only. However, the baking experts at KAF, always helpful, advised me years ago that KAF All Purpose flour sold in supermarkets is the same as Galahad. This is the red label, not the organic variety. The organic has a different protein content.
Finally, check around the US. Artisan flour is a dramatically improving industry in the US. That is also the case in France. For a long time, the general quality of bread and flour was low. I remember French bakers 40 years ago telling me they were importing flour from the US! Many people credit the critical writings of Steven Kaplan with starting a turnaround in quality and quality contol of flour, thus allowing bakers to do better as well.