I have gotten the Mountain Mist Blue Ribbon batting most recently at JoAnn Fabrics online when it was on sale. Not sure what the local JoAnn store carries now as haven't been there in a long time. Also try Googling for online sources. Another batting I like a lot and use is Hobbs 80/20.
I don't prewash my fabrics, so normally there is enough sizing already that I don't have problems with distortion during cutting and piecing. If I do have to prewash a fabric (say, to set dyes in it), then I starch the fabric before cutting. I also always heavily starch backing fabric and bias strips (even if fabric has not been washed) using a 1:1 solution of Sta-Flo and water; the heavy starching ensures stability.
Many people spray starch as they piece blocks. I don't, but that's because I personally have difficulty with spray starch. Seems like the overspray gets everywhere, plus I don't have the patience to wait for starch to sink in and my iron is always too hot so I have a tendency to scorch the starch.