Whole Foods is about your only option for now. Their halibut is very good.
Albertson's on the Old road has a seafood counter where you can pick out what you want, but I don't know about the quality.
Valencia is getting a Bristol Farms, so when that is done you could try there. There are a few meat markets, but I don't believe they have seafood.
A search in Yahoo yellow pages revealed one in Sylmar, but I haven't been so I can't tell you for sure.
Bayshore Seafood Market
(818) 899-5347 11936 Foothill Blvd
Sylmar, CA Map