I don't have a website to suggest, but NC Science Olympiad has a really great setup made from PVC pipe and a water cooler bottle. It looks easy to make and I've always thought they should make the plans available and use it as a fundraiser. It sends the sand down a tube into the bucket and also cuts off the flow when the tower breaks. Contact Jason @ http://www.sciencenc.com/
Also Balsa Man who posts here regularly has a totally awesome structure tester which allows you to test to failure rather than to destruction. It saved us tons of building hours to use this. Basically it is a tripod tee-pee which suspends the chain from the apex and it goes down to hold the bucket. just at the loading block, you put a coupler with eye-loop bolts which allows you to limit the amount of deformation on the structure. Terrible explanation but awesome project. Search Balsa Man's postings for photos and a better description.