See here where they the main ingredients as
Water, Oak Gall Nut, Gum Arabic, Soot, Logwood, Copper Sulfate or Iron
Sulfate. There are many recipes for this ink and many laws the Scribe
must follow to produce kosher ink. There is a recent movement among
Soferim to stop including the sulfate, either iron or copper, in the
ink. They are giving two reasons for this decision. First, the
commandment that no product ever used for war, including iron and
copper, can be used in writing a Sefer Torah. The second reason is the
quick decay of the scroll, 150 years maximum, due to iron gall ink
degradation. Many Soferim are simply going back to the soot base ink.
It is the sulfate that causes the chemical reaction which results in
the degradation of the ink and it is the quality and amount of gum
Arabic that causes the ink to crack and fall off the letters.
Nonetheless, the article continues to list a different series of ingredients used more often today:
- 3 oz oak galls
1 oz logwood shavings
2.2 oz gum arabic
1.9 oz copperas
Rabbi Abramowitz translates the Rambam (1:4) Tefillin, Mezuzah v’Sefer Torah
כֵּיצַד מַעֲשֵׂה הַדְּיוֹ. מְקַבְּצִין הֶעָשָׁן שֶׁל שְׁמָנִים אוֹ
שֶׁל זֶפֶת וְשֶׁל שַׁעֲוָה וְכַיּוֹצֵא בָּהֶן וְגוֹבְלִין אוֹתָן
בִּשְׂרַף הָאִילָן וּבִמְעַט דְּבַשׁ וְלוֹתְתִין אוֹתוֹ הַרְבֵּה
וְדָכִין אוֹתוֹ עַד שֶׁיֵּעָשֶׂה רְקִיקִין וּמְיַבְּשִׁין אוֹתוֹ
וּמַצְנִיעִין אוֹתוֹ. וּבִשְׁעַת כְּתִיבָה שׁוֹרֵהוּ בְּמֵי עֲפָצִים
וְכַיּוֹצֵא בּוֹ וְכוֹתֵב בּוֹ. שֶׁאִם תִּמְחָקֶנּוּ יִהְיֶה נִמְחָק.
וְזֶהוּ הַדְּיוֹ שֶׁמִּצְוָה מִן הַמֻּבְחָר לִכְתֹּב בּוֹ סְפָרִים
תְּפִלִּין וּמְזוּזוֹת. וְאִם כָּתַב שְׁלָשְׁתָּן בְּמֵי עֲפְצָא
וְקַנְקַנְתּוֹם שֶׁהוּא עוֹמֵד וְאֵינוֹ נִמְחָק כְּשֵׁרִיִם:
Ink is made by gathering ingredients like oils, tar, wax, etc., and
kneading them with tree sap and a drop of honey. It is moistened,
crushed into flat cakes, dried, then stored. When one wants to write
with it, he soaks one of the cakes in gallnut juice or something
similar, after which he would be able to write with it. If one tries
to erase it, it would erase. This is the most desirable ink to use
when writing sefarim, tefillin and mezuzos, but if one use gallnut
juice or vitriol, which cannot be erased, it is valid.
OU offers an additional shiur on the topic as well.
As far as purchasing the ink, there are a couple of websites offering already made ink. For instance, this site offers 200 grams of ink for about $25.