What concentration of sulfur hexafluoride is safe to inhale
I'm afraid this answer doesn't address where you can get $\ce{SF6}$, as I don't know any suppliers myself (it would probably help to specify where you are, though). I just want to straighten out the safety details and make sure you (and anyone else who reads this in the future) know what you're doing. All chemistry experiments can take a turn for the worse if done without adequate preparation.

Pure sulfur hexafluoride is a rather chemically inert gas (unlike the related but very toxic sulfur tetrafluoride $\ce{SF4}$ and disulfur decafluoride $\ce{S2F10}$), and it can be safely handled in regular situations. Decomposition of $\ce{SF6}$ usually requires aggressive conditions, such as exposure to molten sodium metal at 200°C. There are some situations where $\ce{SF6}$ reacts at low temperatures, but that requires irradiation with deep-UV photons or direct contact with solvated electrons from alkali metals dissolved in ammonia. I don't suppose you'll have to worry about such situations. It also seems that $\ce{SF6}$ can decompose into a small amount of the other aforementioned toxic sulfur fluorides if it is exposed to electrical arcs, but since it is an electrically insulating gas, the electric fields required for arcing are even higher than in air, so this also shouldn't be a problem.

All in all, both pure $\ce{SF6}$ and helium are very unlikely to cause chemical damage to the body, even indirectly. However, that doesn't imply they are completely safe. Of course, they can still act as an asphyxiant simply by displacement of oxygen in the lungs (make sure to read this!). Be aware that the brain is incapable of directly measuring the oxygen content in your blood, and instead measures oxygenation rather indirectly, by the amount of carbon dioxide. It is possible to faint and even die from hypoxia without any warning. Additionally, since $\ce{SF6}$ is relatively heavy, it requires more effort from your diaphragm muscles to expel from your lungs, making $\ce{SF6}$ inhalation slightly more dangerous than helium.

As a safety measure, do not take consecutive breaths in the non-oxygenated gas and do not spend more than 5 seconds without inhaling air. While your lungs are devoid of oxygen, not only are they unable to transport oxygen into your blood, the lungs actually work in reverse and help expel oxygen from your blood. After performing the trick, take deep breaths in regular air for a minute or two before trying it again, to be sure all the previously inhaled $\ce{SF6}$ or helium has exited your lungs and your oxygen supply has returned to normal. Hyperventilating before or after inhaling $\ce{SF6}$/helium is likely a bad idea, as it further messes with the balance of oxygen and carbon dioxide in the blood. You should avoid any possibility of becoming light-headed and fainting, especially if you decide to do the trick multiple times. Also be mindful that the gasses can accumulate in poorly ventilated spaces, with $\ce{SF6}$ being a particular problem as it will tend to stay close to the ground and disperse with more difficulty. 

If you're going through the trouble to get $\ce{SF6}$, you should do the floating aluminium boat display, too. It's pretty cool! Here's another video showing you can even pour the gas into the boat and make it sink.