The seal on my coolant tank is badly tattered, so a new one is in order. Question is - should I buy a cap from the dealer, or will an aftermarket (from a local auto parts store) be sufficient?

I am going to the local MB dealer tomorrow to get an oil filter (changing tomorrow) and need to know if I should get a cap from the dealer. I am not sure, but it may be an item that needs ordered.

Thanks in advance.