It´s great when stuff comes free, isn´t it! I have to raise beds due to a major molerat and boar problem. I wanted to use Straw bales but they cannot be had for love nor money round here. I am thinking of using uralita, asbestos free, of course. In the UK we used to go around the back of the diy stores near the loading bays and often found broken pallets there. Places like aldi, Lidl and other supermarkets may help too.