1) Try the petrol station closest to the city centre at Av de Navarra/Calle de Zemoriya. Hitchhikers have found French cars here.
2) Take local transport (bus or train) to Irun and walk towards the border. After crossing the bridge into France take the highway right along the river, to the péage. It's easy to go to Bordeaux. Stand just after the péage because drivers will be less concerned and quicker to pick you up after crossing the border.
From the Irun motorway toll-gate, it should also be possible to find a ride heading into Spain or to Portugal.