Hi, I live in France, and you can buy quite a few things in the supermarkets, although you do have to scour for them a bit.
*'Son d'avoine' is oat bran and you can get this at most supermarkets (not the small village ones). It can be found in the 'dietique' aisle (with health foods)
*Germe de Ble is wheat bran (NO... sorry but it's "son de blé", not germe...), again in same aisle as above.
*Skimmed milk is 'ecreme' and you can only normally find longlife milk (it's ok when chilled)
* Cottage cheese is really hard to find. The only place I've found it is in a 'Geant' hypermarket. It is called 'cottage cheese - recette anglaise' and is made by Jockey (in a green packet). It is made with skimmed milk (ecreme) and is 3.9% fat for 100g (2 x 200g in a pack)
*Quark - I use '0% Fromage Blanc'
*Yoghurts - unfortunately I've never found a low fat Greek yoghurt, and Muller lights don't exist (sniff, sniff), but you can loads of 0% yoghurts - brands include 'Taillefine' and also 'Ferme et Fondant' (which is what Dukan recommend), all in a variety of flavours.
*Cheese - there is generally a small section for low fat cheese - Bridelight do a low fat cheese similar to 'Vache qui rit/Laughing cow' and also a hard cheese.
*Creme Fraiche - again Bridelight is only 4%
*Rhubarb - as this is a seasonal product (and French supermarkets tend to only sell seasonal fruit & veg),you can only get this frozen - Thiret or Piccard are frozen food shops.
*Tinned sardines - you can get some lovely tinned ones that are in lemon and basil (no oil), which are lovely on the Dukan bread (which I grill or toast so that it is slightly crunchy.
If there are any other things you need to know, I'll try to help out!