The Shimano boss adapters seem to be sized for unpainted bosses, so they're almost always tight as buggery.

Sometimes they're so tight that no amount of prying will do it, and you just need to grab em with visegrips. You should be able to twist them a degree or two back and forth, then they'll come right off.

Doubt you can find the part as a listed spare, but a good bike shop will have a stash of these I'd say.