Alternative to the C02 cartridges:

Instead of using the CO2 cartridges that can become quite expensivel you can also get a large C02 bottle that you can refiill. The initial bying of the bottle might be a bit of cash, but refillls are definitely cheaper then buying seperate C02 cartridges. You also require a CO2 Gas Bottle Regulator. You also need a hose and clamps.

You can use the CO2 to Force carbonate your beer.

If you do not want to handle or buy a complete new CO2 Gas Bottle (searching someone that refills it often depends on where you bought it) you can use Soda Stream (or compatible) CO2 cylinders. These are much cheaper than cartridges, available in a lot of stores (also online of course), safe and easy to use. On top of that you can swap them in the store when empty.

WARNING: keep in mind that SodaStream uses different connectors for EU / US (Tr21-4) and Down Under (G1/2)!

You can use the standard Mini CO2 Regulator, buy a matching adapter to the soda stream CO2 Cylinder (or from China) and to avoid any leakage of beer into the CO2 Cylinder you also should use the corresponding tubing with valve disconnect.

Alternatively, you can also buy a separate regulator with gauge for more precise feedback.