I made me own for my seriesIII. First one I made was made from a Halfords trailer cover tarpaulin cut down a bit and with eyelets I bought from the local upholstery supplier, lasted about 6 months. Second one was made from a piece of lorry side, those curtain siders, cut to size, glued and with the eyelets left over from the first one.
The pick up bed has everything you need to tie it down, first one I used a bit of nylon rope second one used elastic rope.