The Repsol gas man in Tortosa parks his wagon by the rowing club,down by the river (where else) opposite the police station.You will need a gas certificate which means you will have to get a  registerd gas fitter to check and install your gas appliance.He will then issue you a certificate and you take this to  the gas shop opposite the gas man.He then registers you and you will be asked back the following day,if you're lucky,to pick up your cylinder.You could,if you choose to,is borrow a friends certificate.(Not my suggestion but the gas mans,after I'd registered of course......)Cost about 20 - 25 euros for a first full cylinder,you then exchange the empties for fulls for about 12 euros.
The only person that can speak English is a man in the repsol office in Tarragona.

we were very lucky with this. we went to the repsol office and bought a portable gas heater. Once we had done that we explained that we did not have any gas bottles- and the guy behind the desk did the contract for 2 bottles there and then. it was just a matter then of visiting the bottle man to collect them.  Of course, this doesn't do anything to check your gas installations - so these should be checked out first.
Regards, Haze and Alan