The surfing spot is in the southwest corner. As you enter from Castelia city there are two buildings on you immediate left. Go between them and go westthrough the sand until you see stairs and go down them and you will see a surfing spot.
You can also find Routes 4, 13, 17, 18 and 21, Virbank City, Virbank Complex, Undella Town, Undella Bay, Seaside Cave, Humilau City, P2 Laboratory in B2/W2.
Source: strategy guide and http://bulbapedia.bulbagarden.net/wiki/Frillish