I have wanted to learn free motion quilting for some time now. Leah Day does a lot of FM quilting & a lot of "filler" work. Several years ago she created 365 different FM filler designs in one year. Here's a link to 450 of her designs & videos of how she makes them.
http://leahday.com/pages/quilting-design-gallery
I think FM quilting takes a lot of practice which I'm not good at. I just don't like to practice. I have tried a few things on doll quilts, small bags, etc. but never have gotten good at it.
Now for your first question, I don't know of any place that sells pre-made sandwiches but am interested to see if anyone comes up with an answer for you.