The size of the helium tank you should buy depends on how many balloons you will want to launch. You can buy a small portable helium tank for about $20 at most party supply shops. We found our first small tank at Target. These small non-refillable tanks will only fill about 10-15 16" balloons.
We have found it more economical to go to our local bottled-gas company and lease a refillable 4 ½ ft.(medium sized) tank. It's a little more difficult to handle because of its size and weight (Approx. 75 lbs), but it will fill about 40-50 balloons. Because the limited volume of helium in small non-refillable tanks, we feel they are a very poor choice if you wish to pursue this hobby for any length of time. We have found that a 4 ½’ or 5’ refillable helium cylinder from your local bottled-gas company works the best for us.
I wasn't able to located a good site about helium safety, however you should remember that a helium tank is under pressure and must be handled with care. Do not drop the tank and make sure the nozzle is on tight. Make sure the protection lid (used on larger tanks) is secure when not in use. DO NOT INHALE HELIUM!
If you have any questions or comments about helium, please e-mail me.