some outdoor carpet works fairly well when wet and soapy. Some people have said that a wet tarp slides well, but i've never tried it. In my experience, astro turf with soap and water slides the best.
Buy a snow cone machine and lots of ice. You can have an inrun built after about 12 hours of ice shaving. Make sure you have some portable air conditioners so the artificial snow does not melt.
ice. the stuff they use at the water ramps at lake placid is like this rough carpet stuff. if you can find it and water it it should work for you. you could probably buy it somewhere.