I've bought poly pellets at Walmart.com a 10# box is just under $23. I also ordered from TailorSpot on Etsy, those were the heavier pellets.
I've made 3 blankets so far (I made one today for a friend.) but don't piece the fronts I use a solid piece of fabric, usually flannel or minky.
I use a piece of pvc pipe or a cardboard wrapping paper tube stuck down in each channel and a small funnel to pour the beads into the tube. After each row is full I move those beads down as close to the previous row as possible and then put in a row of pins to hold them in place while I take the blanket to the machine to sew that row closed.
These blankets aren't hard to make but they get quite heavy to move around.