SD Wholesale Seafood Market??
I haven't been to that place in LA, but Catalina Offshore Products is a major wholesaler; they will sell to consumers in bulk through their website, and locals can go down to the warehouse to pick up instead of paying for delivery. http://catalinaop.com/
They do have a minimum order; is that okay?
re: Captain Jack
Google Maps is the best.
I can't speak from personal experience, but I hear that live uni is clearly superior. I'd get a whole urchin and pop it open in front of the guests. At only $15 for 3 lbs it would be cheaper than at most restaurants.