Reddit mentions of Venzo 10x Mountain Bike Tire Inner Tubes 26x1.5/1.75 F/V

What u/phatbrasil said.
The reason a neckweight is the best, is that your positive buoyancy comes from your lungs. The rest of your body is neutral. So the weight to counteract that positive buoyancy should be placed close to the lungs, or else your position in the water will be skewed.
If you have perfect, neutral buoyancy, but skewed, you will be correcting your position with every stroke. that's a waste of energy, and one more thing on your mind. And the whole point of freediving is to be able to forget EVERYTHING but the feeling of total freedom :-)

Pro tip: when you fill the inner tube with lead shot, use a tube for mountain bike tires ( http://www.amazon.com/Venzo-Mountain-Inner-Tubes-26x1-5/dp/B006WRVY9Q)
That should be able to hold at least 2 kg.
If you're looking for more weight, either find a bigger tube, or do this:
when you've poured the maximum amount of lead shot into the tube, close the end you've poured into as well ( with a zip tie, or a clip of sorts) Then start massaging the lead shot towards the other end. some use compressed air to blow up the tube, and then the lead shot just falls into place, and you can pour in some more. I've managed to fill 4,5 kg's of leads shot into a mountain bike tire tube.
Only remember to use some duct tape to stabilize the tube afterwards, or else it will break eventually. And you dont want to spend an hour picking up lead shot from the floor of the local swimming pool. (although it was good apnea exercise :-) )