try the builders yard on the island in Mazarron town not port.
should be a lot cheaper and a lot of choice on colour and size.
If you are going from Condado take the final exit (3RD) exit 25 off dual carraige way.Turn left
Its the road that takes you down to Bolnuevo, think theres a digger on the island or roundabout
it will be on your left so you will have to turn left at island. Its the end one. think i paid 2-3 euros a bag
wanted about 10-12 euros in garden centres.