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.
Another suggestion was brought by Yitzchak Goldhar in his book Admat Kodesh, pp. 58-59. After exploring southern Syria (the area of the Gilead), he concluded that after Yiftach defeated Amon, he settled in a city named after him in Gilead. Near that city was another one called Shukmazai (שוקמזיי, sometimes also called שוקמזון), which may have been named after Yiftach's daughter. In other words, Yiftach's daughter was called Shukmazai. It's important to emphasize that this is entirely a sevara and has no apparent basis in other sources, although online there's repeated mention of this idea with the claim that there are (multiple) old sources that do say this. I have yet to find anything older than Admat Kodesh.