What is the source that the תרמודאי were wood merchants?

Question

In gemara Shabbat 21b it says

עד שתכלה רגל מן השוק ועד כמה אמר רבה בר בר חנה אמר ר' יוחנן עד דכליא ריגלא דתרמודאי: The expression until traffic in the marketplace ceases is mentioned here, and the Gemara asks: Until when exactly is this time? Rabba bar bar Ḥana said that Rabbi Yoḥanan said: Until the traffic of the people of Tadmor [tarmoda’ei] ceases.

on which Rashi comments

רגלא דתרמודאי - שם אומה מלקטי עצים דקים ומתעכבין בשוק עד שהולכים בני השוק לבתיהם משחשכה ומבעירים בבתיהם אור וכשצריכין לעצים יוצאים וקונין מהן: They sold kindling wood and remained in the marketplace later than everyone else. People who discovered at sunset that they had exhausted their wood supply could purchase wood from them.

I would like to know what the source for this explanation is. To my knowledge there is no other reference to תרמודאי as wood merchants in the Talmud (having checked Sefaria, the Jastrow dicitonary and the sefer haArukh).

Answer 1

It's practically impossible to know what Rashi's exact source was, but a few explanations of the word תרמודאי surrounding wood and merchants appear in Otzar HaGeonim, Shabbat pg. 30, so that connection is documentably older than Rashi and his having seen or heard of such a tradition is not surprising.

Answer 2

mevaqesh commented on this question:

Rashi was the recipient of oral traditions about the Talmud, see http://judaism.stackexchange.com/a/68034/8775. accordingly, sometimes his sources are not textual.

Therefore, a reasonable null hypothesis is that Rashi's source for the identification of תרמודאי is something he learned orally from his teachers in Worms and Mainz.