Your Spanish friend might have meant Dam in Amposta.  I get mine from there.  It's near the river by the N340 bridge.  I dilute mine at 80:1 i.e. 120ml to 10 litres water.  10 litres (a backpack full) will treat about 1,000 sqm at a cost of about 75 cents.
All the builders yards will deliver for a charge (including Mayor) so the crack is to buy everything you need now or are planning to need in one go to dilute the delivery cost.  That's why most people have a permanent pile of sand on their property for example.