From where did they obtain the stubble?
Oznayim la'Torah #1: They searched all over Egypt, as the Pasuk states, and not just in Goshen, where they lived. 1
Oznayim la'Torah #2 (citing Tosfos to Bava Metzia 103): They collected the stubble from underneath the straw in the fields of the Egyptians, who did not mind them doing so. 2
It is impossible to understand however, how under such circumstances, they could have come even close to fulfilling their quota of bricks. See Oznayim la'Torah to 5:13, who citing his brother-in-law, explains the Pesukim 5:12-18 based on this question.
Perhaps they were even happy with the situation, since the Ivrim were cleaning up their fields and preparing it for the next harvest. According to Tana de'Bei Eliyahu however, an Egyptian who found a Yisrael in his field collecting stubble, would beat him - See Oznayim la'Torah.
