I'm wanting to pinpoint this hadith, verify it's legitimate, and check that the translation is accurate.
In principle, I could track down Al-Kabeer and identify what it says. However, I don't believe I have access to Al-Kabeer, nor do I speak Arabic. Googling site:sunnah.com lesbianism doesn't give any hits.
You are correct you don't have access to al-Kabeer -in the meaning you stated- and once you may find a book with this name you'll find it is rather unclear reference as you'll need to know the author of the book too.
Imam at-Tabarani has three major works of hadith (encyclopedia's one could say) which are quoted in the Wikipedia article (however the explanation as given there and quoted here seems to me wrong):
al-Muʿjam al-Kabīr المعجم الكبير – from which he excluded the traditions of Abu Hurayra
Al-Mu'jam Al-Awsat المعجم الأوسط – which contains traditions from Abu Hurayra
Al-Mu'jam As-Saghir المعجم الصغير – which gave a hadith from each of his masters..
(Wikipedia on Imam at-Tabarani)
Note that al-Kabir means the big (the great) in Arabic, while al-Awsat means the middle and as-Saghir means the little or the small.
Narrations with a similar wording
Now according to the Islamweb library my first attempt to find a hadith with this wording or meaning was that the hadith you are look for is not even quoted in at-Tabarani's al-Kabir but in al-Awsat. Note that al-Haythami in his Majma' az-Zawaid quoted the hadith and said it is compiled in al-Kabir and in al-Awsat both on the authority of at-Tabarani's teacher Ali ibn Said ar-Razi علي بن سعيد الرازي who has a bad reputation and is considered as lenient, while the rest of the narrator chain is strong and thrustworthy.
Here's the quote from al-Awsat on the authority of abu Musa al-Ash'ari أبو موسى الأشعري:
" لا تباشر المرأة المرأة إلا وهما زانيتان ، ولا يباشر الرجل الرجل إلا وهما زانيان " .
- - - (My own translation take it with care)
No women sleeps (copulate) with an (other) women unless if they are (considered as) -two- adulterers (zaniyatan) and no man sleeps with a men unless they are (considered as) -two- adulterers (zaniyan).
Al-Bayhaqi also quoted a similar narration in his as-Sunan al-Kubra (also on the authority of abu Musa al-Ash'ari):
إذا أتى الرجل الرجل فهما زانيان ، وإذا أتت المرأة المرأة فهما زانيتان
“If a man has sexual relations with another man, they are both guilty of zina, and if a woman has sexual relations with another woman they are both guilty of zina.” (Source of the translation: islamqa #21058)
Here again al-Bayhaqi considered a narrator called Muhammad ibn Abdurrahman محمد بن عبد الرحمن as unknown and therefore the hadith with the given chain as rejected (note that this person starts the part of the narrator chain which differs from at-Tabarni's narration).
The hadith from al-Kabeer and an other narration
The statement of al-Haythami and the fact that the sahabi on whom's authority the hadith of the fatwa has been narrated made me look further and finally I found in al-Kabir (or al-Kabeer) on the authorithy of Waatilah ibn al-Asqa' واثلة بن الأسقع الليثي (who died around 83 a.H. with an age of 100 or more years according some narrations):
السحاق بين النساء زنا بينهن
And al-Haythami has also quoted it in his Majma' az-Zawaid and commented that the narrators of the chain are trustworthy.
Finally there's an other hadith compiled in at-Tabarni's al-Awsat and al-Bayhaqi's sho'ab al-Iman on the authority of Anas ibn Malik ():
إِذَا اسْتَحَلَّتْ أُمَّتِي سِتًّا فَعَلَيْهِمُ الدَّمَارُ : إِذَا ظَهَرَ فِيهِمُ التَّلَاعُنُ ، وَشَرِبُوا الْخُمُورَ ، وَلَبِسُوا الْحَرِيرَ ، وَاتَّخَذُوا الْقِيَانَ ، وَاكْتَفَى الرِّجَالُ بِالرِّجَالِ ، وَالنِّسَاءُ بِالنِّسَاءِ
- - - (My own translation take it with care)
If my Ummah made halal six things they will be doomed: If appears among them the cursing (curse each other), and they drink wine (alcohol), and they wear silk, and they had female singers (coiffeuse ?), and men were satisfied with men and women with women"
Interpretation of this Hadith
In his al-Mughni ibn Qudamah said (9/59):
If two women engage in lesbian sexual relations, then they are guilty of zina and are cursed, because it was narrated that the Prophet (peace and blessings of Allaah be upon him) said: “If a woman has sexual relations with another woman, then they are both guilty of zina.” There is no hadd punishment for them because they did not have intercourse, rather it is likened to intimate relations that are less than intercourse, and they are to be subjected to a ta’zeer punishment. (Source: the fatwa quoted above)
So Lesbianism is set equal to adultery, but without the same shari'a punishment (in this life)!