Sforno on Leviticus 24:5/ספורנו על ויקרא כ״ד:ה׳

1ולקחת סולת, this commandment presumably also applied only after the initial donation of such flour which had been contributed when the Tabernacle was being built had been exhausted. That original amount had been included in what was mentioned in Exodus 39.33. [one wonders where all the flour came from during those years when the Jews, in the desert were cut off from contact with the surrounding nations so that even their exact location was unknown. Ed.]