What the Bible says about East of the Jordan
(From Forerunner Commentary)
Where did this food come from? It could not have been manna because manna could not be stored. It could have included some meat they slaughtered from the herds and flocks traveling with them. It could also be evidence that the Israelites were already gathering and perhaps eating the produce of the lands they were conquering. The narrative does not say what lands, but it could include the Promised Land because people were already beginning to take up residence east of the Jordan.
John W. Ritenbaugh
Countdown to Pentecost 2001