I know that these do work. I know a few people that use these for their kids. I tried to look for it in bulk and I had a hard time finding it also. It can be so pricey for them if you can't get them in bulk.
I would not put anything in it that is not washable. I have seen the pattern online about sewing channels in them and rolling sheets up and sticking them inside. My friend tried it on her son and he didn't like it as much as the weighted blanket that she bought him. I told her that if she can find the beads, I'd make his next one for him. They do also make smaller ones that just go on your shoulders(used for reading, school time, playing). I don't know if that would be an option for you to make instead. Good luck on your search!!! If I find a good price on them, I will be sure to let you know.