Hello. Fleece wicks away wet so is a good top for a liner. You can use wadding from sleeping bags or bed covers from charity shops, they are only going to wee on it after all. I use wadding (sleeping bags etc) underneath and then a separate top fleece bit so that I can wash the padding without putting loads of hair into the washing machine. I have made liners for charity which are fleece + wadding + fleece in a sandwich, but I don't use them personally. They might suit you better though.
If you are after buying the batting then I am not sure. I recycle rather than buy. But whatever a bed throw is just double that, or whatever a sleeping bag is that will do. I'm not sure if this helps, I hope it does!