The only source for Mithril is Moria. Any mining going on there lately?
Gimli was hugely impressed to just be able to look on a Mithril shirt that Frodo was wearing.
But obviously, by the way he recognized it, Gimli had seen Mithril, right? Which means there's more out there.
I would just simply question the quality of Mithril offered by a Gondolier...
Not hugely impressed (as if he had never seen it before). More like proud. All he said was, "You are full of surprises, Master Frodo", with the same pride you would if you found one of your children valuing that which you also hold dear....