Hi
Not sure from your post if you intend to start out from the airport, which you can do as the camino passes about 1 mile from the entrance. If you do then you can try these numbers, Biarritz tourist office 05 59 22 37 10 or Bidart torist office 05 59 54 93 85 they may not keep credencials but they may be able to point you in the right direction. The bidart office had a stamp with the scallop shell on it to indicate its links to St james so this might prove more useful.
There is also a gite pelerin about 10 km before Hendaye in Urrugne, it only opened in April, they may have a credencial, I stayed there and the welcome I received was one of my highlights. Their details are Bertrand 05 59 54 33 18 or 0631 68 81 17/ Jean Pierre 06 30 49 70 54. Jean Pierre speaks limited english so if you explain your situation they may be willing to help you.
Mike