There are 30rd metric mags around. I've got one kicking around at home somewhere. However I believe most that you find came from South Africa. They're straight (not curved) 30 rounders and were actually designed for the SA 7.62 converted Brens, not the FAL but as they have the front metric "beak" rather than the inch pattern rectangular "tab", they're interchangeable with FALs and 20rd mags.
There's the heavy barrel FN FALO/FAP too as used by the Israelis and Argentinians but I believe they utilised 20rd mags as standard.