I understand that there is a tradition that the Third Temple, may it come swiftly, will be built in Heaven and delivered intact to Jerusalem by G-d when the Messiah comes. What is the source for this view?
A few of the sources are:
Zohar based on Chagai 2:9 גָּדוֹל יִהְיֶה כְּבוֹד הַבַּיִת הַזֶּה הָאַחֲרוֹן, מִן-הָרִאשׁוֹן says that the third Bais haMikdash will be built by Hashem.
Yalkut Shimoni Tehilim 848 on Tehilim 93:5 עֵדֹתֶיךָ, נֶאֶמְנוּ מְאֹד--לְבֵיתְךָ נַאֲוָה-קֹדֶשׁ: ה', לְאֹרֶךְ יָמִים explains that when the house of Hashem will be built by Hashem then it will last forever.
Look in Tanach with commentaries -- Yechezkiel chapters 40-48. Pretty explicit.