It is strongly recommended that you fill the balloon with helium. Helium is safe because it does not burn. Some people use hydrogen but it is extremely flammable. If it becomes mixed with even a small amount of air it is incredibly explosive. Stores selling party balloons do not use hydrogen for safety reasons (but always be sure to ask). Hydrogen also requires you to buy your own tank and regulator which are an added expense.
The most commonly used balloons are Mylar party balloons that are approximately 3 feet across. They are usually clear or silvered but decorations printed on the surface are fine. Do not use rubber or latex balloons. Helium leaks through them very quickly. If they are filled the day before they will have lost most of their helium when you are ready to fly them. Any store selling party balloons will fill a balloon for you. If you cannot find a store that sells the large 36 inch balloons you can buy several smaller balloons and tie them together.
The balloon(s) should have a lift of approximately 70 grams or more. The transmitter and GPS weigh less than 40 grams so the overall net lift is approximately 30 grams or more.
You can also buy and fill your own balloons. Qualatex foil balloons are a good choice. On Amazon look for: “Silver Giant Round 36″ Qualatex Foil Balloon”. You will also need a tank of helium and a regulator. Typically you can buy the tank or rent it on a monthly basis. I have found that prices for helium vary tremendously from one supplier to another. It is probably worth calling every industrial gas supplier in your area to find the best pricing. A hose and an air nozzle attached to the regulator make filling easier.
Since the helium in the balloon will expand as the pressure decreases at higher altitudes you may not want to fill the balloon completely full. Filling the balloon until it has a lift of 60 grams is a good starting point. Once filled the neck of the balloon can be sealed in a couple of ways. Using a heat sealer for Mylar bags is very common and works well. Inexpensive heat sealers can be found on Amazon. Twisting the neck of the balloon and then putting a zip tie tightly around it also works.
Helium is also available in smaller tanks of “helium balloon gas”. These may be available locally or can be purchased on Amazon. Note that “helium balloon gas” is not pure helium but has approximately 20% air or oxygen added to it. As a result the lift will not be as great as pure helium and more than one balloon may be required.