No strict rule, just the narrative approach (as detailed here) and common sense. If the narrative has your magus making friends with a Bjornaer magus and trading to get access to Voic of the Bjornaer, for example, that is fine. Want to visit Durenmar to search the Great Library for a variant spell? Certainly possible but probably have to ask a few years in advance, pay the visitor's fee and spend a season locating the desired lab text. Of course, as a Mercere magus you can rely on the redcap network and House library (which is probably specialized in a few limited areas), but you shouldn't abuse it.
If it's in one of the official published (see here for a hierarchy of the books), it is probably possible to find a lab text somewhere. But it should probably require some time to locate it, and may require some trading.
If in doubt, just ask. 