Regular cotton batting (i.e. what we use for quilts) generally contains glue and resin. It is glue and resin that can cause fires in microwaves.
I only use the Warm Tater Batting that is made especially for microwaves as it is glue and resin free. I have found the best deals for this at Fabric.com. I watch for it when it is on sale and then order. I do a lot of craft fairs in the fall. Last weekend I sold almost 20 of them and came home with an order for 10 more for Christmas gifts!