What kind of batting would I use if I am making a baby blanket or comforter with little or no quilting? Where do you get yours?

If you're going to do little to no quilting I'd use a flannel sheet.

Or Warm & White.

The fact that you mentioned comforter makes me think you want it puffy so I would actually cut a piece out of an acrylic blanket and use that as batting. I just bought a queen size acrylic blanket from the clearance aisle at Walmart for $7. I bought it just to try using as batting. Right now it is folded across the bottom of our bed to keep out feet warm but I eventually intend to make a duvet or bed runner to match the quilt that is all ready on the bed.