Sulfur Hexafluoride

I thought this was pretty cool. Apparently, there is this gas called "Sulfur Hexafluoride" (a.k.a. SF6). It's about 5 or 6 times denser than air, (meaning that you can float lightweight things in it,) it's invisible, (increasing the "cool" effect,) and it has some strange effects on your voice.

Here are some cool videos with Sulfur Hexafluoride.
I recommend "Invisible Water" and "The Tonight Show: Jay Leno and Josh Duhamel sniff some gas", but there are some other cool ones as well...

The Wikipedia article on it is also interesting.

Now I just want to know if there's a way I can buy some of this stuff...