We have fish warehouses in the city that restaurants buy from that have wholesale fish and lobster. Other than that be wary of other "discount" seafood because you really need to be sure it's fresh.
It would be better to substitute shrimp or crab than to use the imitation. It's an unhealthy chemical and sugar laden substitute. It would still taste good.
CNN has been reporting that about 1/3 of seafood products are mislabeled. How nice is that! I don't know if they can really fake you out on lobster, though. I know it is expensive on the Maine Coast and that is where it comes from....for the most part. Only kind I ever wanted. I don't eat it anymore.
Imitation is disgusting. They soak cheap fish in lobster water and then add a ton of sugars in various forms to it along with other bits of miscellaneous chemicals. Better off buying plain fish and season it to your tastes.