I would avoid regular shop bought brands of cod liver oil. You could try Nordic Naturals. Their oils are molecularly distilled so supposedly much less risk of toxic metals though with all cod liver oils I suppose there may be some small trace. I know people who are quite mercury toxic who take NN CLO.
From what I've heard NN seem to test down to a lower level than other brands, are more transparent in terms of their testing and are 3rd party tested....