There's all sorts you can choose from. Some members on here sell it also.
I buy Salmon Oil from OnlyNaturalPet.com
Only Natural Pet Pure Icelandic Salmon Oil for Dogs & Cats
It doesn't smell like fish, so that's a plus. I've only used it for a couple of weeks so I'm not too sure how well it works for the coat... I've noticed her dry flaky skin has gone down a lot though.