Flannelette Sheets for batting
or blankets
Ask and tell everyone you know what you are wanting ... you maybe surprised who has a stash to clear out ... or Granny's stuff they don't know what to do with it.
If you're doing charitable sewing ... many will donate from their stashes ... some will even go out and purchase new, so you can sew, being that they cannot.
And by the looks of it, there are many people here who have incredible luck at the thrift stores, garage sales, CraigsList, etc.
Good LUck!