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You are actually referring to a set of items that are spelled out in a couple of books.
The generic masterwork tool, from the Core Rulebook.
And the additional information and definitions to masterwork tools in Ultimate Equipment.
And, in general, they are legal to purchase, and give a +2 competence bonmus to a skill, but usually only for a limited number of uses, or in limited circumstances. Note that your Boots of Dance (dance slippers?), for example, would give a +2 competence bonus to Perform (Dance), but only when you are wearing them, and they probably wouldn't be suitable footware for most circumstances, just like real-world ballet slippers...