I have been using second hand 2kg fire extinguishers. Last time i bought some i got 3 for about £30 (delivered) off of ebay. Not having any direct connections with someone that can supply CO2 e.g. brewing, welding or fire extinguisher industries, this is the cheapest method i have found in the UK.

As far as I’m aware regs for refillable CO2 cylinders are standardised within Europe (i think the US uses a different size fitting) although i would check the jbl ones berfore buying anything.

I have found that I get 3 to 4 months out of a 2kg bottle on a 120l tank.

Cheers
Neil