Fish307 (upstate NY - see their website) used to carry the Heritage Lakers and spare parts.
However, their current website doesn't even show Heritage Lakers anymore.
You still might want to call them, though.
Otherwise, give Kittery Trading Post (Kittery, ME) a call. They have a new on-line catalog but if you don't find what you want, they also do mailorder over the phone. They sell the tip ups, don't know about parts.
Jeff