The phrase "are mostly available" is correct and usable in written English.
It can be used to indicate that something is generally accessible or present, but not always guaranteed to be so.
Example: "The resources you requested are mostly available in the library, but some may need to be ordered."
Alternatives: "are generally accessible" or "are typically obtainable."
Like much else in "Prozac Backlash," the facts of the Wesbecker trial are mostly available elsewhere.
*Pd,Co, and Pt are mostly available in the UHVAC Angstrom evaporator.
Whatever their provenance, ABC plates and mugs with names on them are mostly available in the United States and in Britain, although the Japanese are very avid buyers.
Although it offers none of Bernstein's own compositions, which are mostly available elsewhere, it captures his versatility in other ways, displaying him as pianist (in Beethoven's Triple Concerto) and harpsichordist (in Bach's Fifth "Brandenburg" Concerto) as well as informal teacher.
On the other hand, aggregate data are rather poor, as they are mostly available in the form of simple cross-sectional fertility indicators at the district level and only for the periods surrounding censuses (i.e., every 5 or 10 years).
To this aim, white noise sources are mostly available on the market and usually to the purpose, whereas the use of generators capable of producing coloured noise (actually, very uncommon) should be advisable in most of applications.
Mostly Mozart is mostly available.
But until recently smarter nosh was mostly available only in restaurants.
Aside from some limited promotional day passes, the indoor water park is mostly available to guests staying at the resort, where starting rates are $109.95 to $199.95.
Also, information on biogas is mostly available only to researchers, not the potential users of this technology.
Some camps also carry in their own Wi-Fi, but that's mostly available for those just within that camp.
