Moxon's is on Lordship Lane. From Google:

Don't forget Forest Hill's own fishmonger who comes up from Hastings every Thursday. Grant Stanley (07971 145164) parks next to William Hill every Thursday from 6:30am - 10am. He sometimes leaves early if trade is poor or he has sold out, so get there before 9:30 to be sure.

If you want something in particular, give him a call a couple of days before and he'll try to get it. He also comes to the Food Fair, but has had to miss the last few due to lack of fish or family issues - his daughter had a complicated pregnancy.