I'd suggest getting some sort of booth setup with some kind of curtain as a door. The arc from an arc welder (such as MIG and TIG) is as bright as the sun, and can cause eye damage easily. If you cannot get a booth, make sure there is some sort of see-through welding curtain. That way you can safely see what is going on inside without being blinded. The arc can also cause sunburns to people nearby it. We have a heavy leather coat that our welders wear when they tig weld for that reason. (we don't MIG weld our aluminum)
(you can see the curtain we use in the pictures I attached, and you can sort of see the booths we have in the background. Sorry for the poor image quality, I had my friend record a video of me TIG welding with my phone, not the best image quality)
As for gas, yes. It does have to be 100% Argon. That can be bought at different welding supply stores. I'm not sure how gas refills go for us because we haven't needed to refill it during my time on our team, but the closest place I know is an Airgas store.
Praxair seems to be a really credible place to buy it too.
http://www.praxairdirect.com/Special...ons/Argon.html
I'm not sure on the price. I found on a website it is $.50 per 100 grams.
Hope this helps!