Hi Colleen, I found this lovely article that shares the pro and cons of different weight you can use inside the blankets - https://www.weightedblanketguides.com/guide-blanket-fillings/ I'm hoping one of them is what you're looking for
Just a guess, perhaps rice or small beans would work. I use rice as one of my ingredients for the heat packs.
You can use rice, but you will have to remove it to wash the blanket.
You can use beans
You could make several long bean bags, just like the ones used on the corn hole game, only longer. These would be easier to take off when you launder the blanket.