where can i buy stuff to make a giant slip and slide?

If you look at this article here it tells you how to make a Slip and Slide very easily for cheap.  If you have the slide already all you need is a tarp and some sand bags.  Or you can just go without the slide and have them make a run for the tarp.  You could try and piece 2 tarps together.  That's what most kids do.  To make these slippery pour baby shampoo on the tarp.  It must be baby shampoo so it won't sting their eyes.  It also mentions baby shampoo is this article here.  Another idea is to buy the Slip 'N Slide Spiral Splasher.  It uses Hydro-Glide Technology for faster Slip 'N Slide action.  

Hope this helps.  Good luck and have fun!