Greg
Another option that could be less expensive would be the bladders from casks of wine. They weigh 50 grams each and displace 4 litres or 4kg of water and they are quite durable.
The hard part is finding cask wine that is to your liking.
Regards,
Phill
Only problem there is you'd have to drink an awful lot of wine to come up with the flotation available in a couple of canoe/kayak float bags
