Do we have any sources for the name of Yiftach/Jephthah's daughter in Shoftim?
She has no name in the Biblical text, nor (as far as I can tell) in any 'canonical' ancient midrashic or Talmudic texts (see Tamar Kadari's article here). The only pre-modern source to name her is, as @Danno commented, Pseudo-Philo, who names her "Seila," i.e. "She'ila" (Questioner). This name has been picked up by a number of modern writers; e.g. Louis Ginzberg, Elma Ehrlich Levinger, Ellen Frankel, Vanessa Ochs, etc.
Pseudo-Philo is a work of "pseudepigrapha", attributed to the Hellenistic Jewish philosopher Philo but not actually written by him. It was probably composed sometime around the end of the first century CE, and probably in Hebrew, although it survives only in Latin. (See, e.g., Jewish Literature Between the Bible and the Mishnah). Pseudo-Philo's treatment of Jephthah's daughter is actually fascinating and a much richer character than the Biblical version — see Cynthia Baker's article, "Pseudo-Philo and the Transformation of Jephthah's Daughter," and you can read Pseudo-Philo's account here.
I came across now an interesting suggestion brought in the book "Hidden Women in the Tanach, vol. 3: Daughters" by Miriam Samuel. In footnote 37 on pg. 229 she brings a suggestion by Rivka Lubitz that Yiftach's daughter's name was "Tanot", as the verse says: "מימים ימימה תלכנה בנות ישראל לתנות לבת יפתח...". The idea seems to be based on midrashim such as the one about Avraham's daughter Bakol (and see here).
The idea, written in midrashic style (this is something Lubitz does often, for some reason), can be seen here.