The Key of Solomon provides instructions for the creation of holy water. (See Book 2, chapter 11 of the Mathers edition.) It is consecrated on the day and hour of Mercury, making it well suited to the Mercurial focus of that grimoire. It can also be used anywhere generic holy water is called for in a spell.
Another option is to simply obtain holy water from a local Church. In fact, there is quite an elaborate tradition surrounding such water. We see it here and there in the grimoires – such as the Sixth and Seventh Books of Moses, which calls for Three Kings holy water (or water used during an Epiphany Mass on January 6th, Three Kings Day). However, we can find much more of this kind of thing in Afro-Caribbean and Catholic influenced traditions such as Palo, Voodoo, Hoodoo, Curandero, etc.
The following information was posted to the EvocationalMagics yahoo group by Aghor Pir. It is compiled from various systems of Palo (Monte, Mayombe, Kimbisa, etc.)
I was taught the following:
Holy Water: Cleansing, Exorcism and Consecrating, Protection from the Evil Eye
Florida Water: Cleansing, Harmonising, Blessing, and Calming spirits
In Palo it also matters where you get your Holy Water when it comes to what you
can do with it.
Holy Water made by yourself or acquired from a local Catholic Church:
Cleansing, Exorcism and Consecrating, Protection from Evil Eye
Holy Water from 9 Catholic Churches: Protection from Witchcraft
Holy Water from 7 Catholic Churches: Spiritual Cleansing (especially in
Holy Water from 5 Catholic Churches: Spiritual Warfare
Holy Water from a Catholic Church on Easter Sunday: White Magic and Healing
Holy Water from a Catholic Church on Ash Wednesday: Protection from untimely death and protection from death magic (includes Santissima Muerte)
Holy water from a Catholic Church on Good Friday: For doing black magic
Holy water from a Catholic Church on Glorious Saturday: Spiritual Cleansing
Holy water from a Catholic Church on Christmas Eve: Prosperity Magic
Holy water from a Catholic Church on Christmas Day: reverse witchcraft
Holy Water from a Catholic Church on New Year’s Eve: to change a situation (for better or worse)
Holy Water from a Catholic Church on New Year’s Day: Prosperity and Wealth Magic
Holy Water from Catholic Churches of 7 Female saints: Domination and Love Magic
Holy Water from Catholic Churches of 7 Male Saints: Spiritual protection
Holy Water from Catholic Church of 9 Female or 9 Male saints: for Black Magic, Separation and Reversal Magics
Also see my blog post conerning so-called ‘Solomonic Omiero’, a very powerful form of holy water.