You have to be careful where you get your fish oil. And yes, there are other oil products (I think flax seed oil may be one) that you can substitute with.
I think Dr Oz did a show one time on proven brands of fish oil and posted it on his web site. It was about a year ago.

Anyway you have to find a reputable brand. If I find something out I will let you know.
Here is what Dr Oz said: http://www.azcentral.com/health/fitn...-oil-tips.html