aftermarket ones you have only a few options:
carbon fiber one in an OEM look or the Z3 fenders (phase 1 or 2)
the phase 1 you can get in fiberglass or metal depending on where you purchase them. the phase 2 (look way better than phase 1.. just cost more) are in fiberglass only (i havent seen them in metal yet, correct me if wrong though) as for sites just do an online search for "z3 fenders" you'll get a run down of sites.
for me i sold my metal z3's cuz i got tired of seeing every other car with them.. thinking of getting the phase 2 ones if they come out in metal (too many people lean on my car) and i get another aggressive style kit. i dont know the cost now for them but i paid $200 for my metal ones at the time (got a discount) and the fiberglass ones are like $125-150 (depending on store)