CO2

If you are looking for CO² for your grow room then the best solution is to go to your local welding gas supply store and rent a CO² bottle. It is cheap for the gas and you only pay a monthly rental fee for using the cylinder. You will need to buy your own CO² regulator in order to control the flow into your grow setup. The welding store will sell these too.

Products like CO² Tabs are not effective because they produce very little gas, which is simply lost due to the constant extraction of air from your growing area.
A CO² cylinder will literally have 500 times more CO² in it compared to tabs.