any washed gravel will do i suppose - but avoid crushed limestone & slate as it's alkali when wet. Have a look at your garden centre at the decorative gravel / crushed stone = lots of choice & it's not that expensive. But wash it thoroughly even if it says it's washed already. Can't u order some clays online? http://www.theonlineelectricalwholesaler.com does 25ltr sacks of proper clays for £11.50 which is pretty fkin cheap imo - wouldn't fancy paying the postage on an item that heavy, but I'm sure u can get some local if u look around. Another great media is charcoal - barbecue stuff - bash it into bits first obviously, then rinse it thoroughly as a lot of it is presoaked with paraffin / wax to make it light quicker. It's neutral in ph & has great water retention properties. Failing that, rip open ur little sister's favourite teddy & use the polystyrene balls *tip: don't get media with too small particles as they could pass through your net pots / drain holes & block up your pumps / pipes = dead plants before u know it. Either that or a nasty flood!