Courtesy of this trip report, the following re-supply points have been identified:
o Hendaye: before starting; several adequate grocery stores are available.
o Arizkun: two small stores with limited, but sufficient selection.
o Les Aldudes: a surprisingly useful quick mart in the gas station.
o Col Bagargui (Irati): very small shop at the ski resort.
o Lescun: a well-stocked medium sized market.
o Candanchu: tiny Supermercado El Bozo (easily missed!)
o Gavarnie: small, but moderately well-stocked grocery
o Salardu: barely adequate small market.
o l’Hospitalet-pres-l’Andorre: barely adequate small market and deli.
o Bolquere: excellent market and great deli.
o Le Perthus: huge stores with everything you could ever want.
Postscript: In retrospect, I didn't use Hendaye, but shopped at St Jean de Luz before starting.
I found some provisions at a bar in Arizkun.
Aldudes had a well-stocked petrol station that was open on Sunday morning.
Col Bagargui had a poorly stocked shop, but was at least open.
Lescun had a good small supermarket.
I went via Col du Somport rather than Candanchu.
Gavarnie had good small supermarket.
Salardu, supermarket was small but OK. I was lucky to catch it open, since it shuts at 1.15pm for the afternoon.
Hospitalet was disappointing. There was nothing open except a hotel.
Bolquere had a good small supermarket that was shut when I arrived, but opened at 4pm.
Arles-sur-Tech had good Spar supermarket that was also shut, but opened at 3pm.
Le Perthus was an absolute grim dump of a town. I would strongly recommend passing straight through without stopping if at all possible. I did eventually find a supermarket that seemed mainly to sell large volumes of cheap alcohol. That tells you all you need to know about the place.