Head to a fabric store- they usually stock polyester beads for stuffing stuffed animals- these are pellets, not fibers.
They are light and won't rot like lentils or beans!
I see you are in Rhode island. Go to a sail maker. they have the machine and heavy thread to re-sew. I used one on the Ct shoreline to sew some heavy webbing. Didn't want to charge me, but i left $ on the way out.
What fill is it? If it is sand, then open up a small part that will close easily. Pour out whatever feels right, and then use Gorilla Glue or some such. Fine sand will come out easily, small hole.