What were the snow loads like this year? There may have been nothing wrong with the trusses. Overloading can be a very real cause of premature failure.
Best case, replace the entire roof system. However, if the failure was due to loading, replacement with the same design may not be prudent. Regardless of code requirements, it may be worth your while to reconsider pitch, cord sizing, and truss configuration. There's nothing against the rules about over-building.
If there is a PE stamped design for the roof, liability may fall back on the engineer if the design did not meet minimum code requirements. Snow load is a design criteria.
As for your lumber company, they probably bought those trusses from a third party manufacturer.
