Step 4: Balloon
I searched a while on the internet to find a really good weather balloon. Since it is pretty easy for me to buy things off Amazon.com, I opted for the "more expensive" yet "more reliable" weather balloon from this trusted source. 
In comparison to other weather ballons. This one is quite small, but it will suit our needs. Here's some specs on the balloon:
Diameter at release = 6 ft. Diameter at burst altitude = 20 ft. Volume at release = 50-100 cuft. Nominal Lift = 3.8 lbs. Maximum Lift = 12 lbs. Neck diameter = 2.8 inches. Neck length = 9 inches. Ascent rate = 1000 ft/min. Burst altitude = 100,000 ft. Material = Natural latex
If you're short on cash, these ballons go for as low as $45 from other places.