Where did the Israelites get their leaven?

Exodus 13:6-7 describes the rules on the use of leaven (yeast) by the Israelites, after they left Egypt. The Baking section of the Wikipedia article on yeast says that yeast was used to cause bread to rise in ancient Egypt.

Here's Exodus 13:6 Seven days you shall eat unleavened bread, and on the seventh day there shall be a feast to the Lord. 7 Unleavened bread shall be eaten for seven days; no leavened bread shall be seen with you, and no leaven shall be seen with you in all your territory. (ESV) See also about chametz. The NIV says ". . . within your borders." The NASB agrees with the NIV, using "borders." The King James uses "quarters," which seems to put a different meaning on it, but the three modern translations I consulted indicate that the KJV is misleading on that point.

I ask the obvious question. Where did the Israelites get new leaven cultures after the yearly Passover celebration, when they were supposed to dispose of all the leaven they had?

I don't know, and I haven't been able to find anything resembling an answer. Perhaps you have one.

* * * * *

April 22, 2009.
gluadys (see comment below) has provided a probable answer, namely that there is some yeast found in the grain, and, therefore, in the flour made from it. The Wikipedia article on "pre-ferment," on this date, says that sourdough starter relies on such wild yeasts (and bacteria). Thanks, gluadys!