You are right. There is no explicit prohibition written in the Torah regarding relations between women. In fact from Rambam's statement in his Commentary on the Mishna (Sanhedrin, ch. 7) that "women having relations with one another is immoral, but there is no punishment for it, neither from the Torah nor the Rabbis" one might think there is no prohibition at all.
However, "no punishment" doesn't mean there's no prohibition. Rambam in Mishna Torah (Isurei Biah 21:8) explains that a Torah prohibition is indeed derived from the verse against following in the ways of the ancient Egyptians: "For women to 'intertwine' with each other is prohibited and is included in the acts of the Egyptians of which we are warned, 'Like the practice of the land of Egypt...you shall not do' (Lev. 18:3). Our Sages say, 'What did they do? A man married a man, a woman married a woman.... (Torat Kohanim 8:8)". The term "marriage" was used only because that was the norm — but the act is forbidden even outside the context of marriage (Tosefot, Yevamot 76).
As a concluding note, it is very important to stress that it was our intention to address only your specific question regarding what the sources say about intimate physical contact between women. We are in no way insensitive to the suffering of people troubled by these issues and empathize with their turmoil, as the Torah commands one to be caring and compassionate to anyone in pain. Also, although we described these issues as prohibitions of immorality, the certain distinction between the act and the person must be made. As in all transgressions, the fact that a person has an inclination to do, or does prohibited acts, does not undermine his or her inherent value as a person.