Barcelona is full of coin dealers (mostly outside the old centre), but about 90% of what they carry is Spanish coins and the rest is mostly Roman and Portuguese. They tend to over-clean horribly. Your best bet may be to enter whatever coin shop is on your way, ask for Islamic coins and if he doesn't have any or only pieces with little interest, ask him if he knows who has them, but make sure you ask him first.
There are two coin markets in Barcelona where they have a bit more variety and also obvious Chinese fakes. See
these posts. However, Sant Antoni (easy to reach by underground) has been fixed up a while back. I saw no coin dealers in the temporary market. Not sure if they have returned since. As a comfort, Sant Antoni is one of the nicest markets in Barcelona anyway. You won't be bored there.
Peter
