The easy way is to get Master of Towing Oceans/NC first, then you are entitled to sit for the Western Rivers Mate of Towing exam without having any Western Rivers “seatime.”
I hear that it’s possible to find Western Rivers Mate jobs, they don’t use the term “Mate” they say “Pilot.” I hear that a necessary phrase to use when you call companies is to tell them that you want to “post for pilot.” I do not have any firsthand experience; just what I hear.