You can sometimes find a gas supplier that will refill a Co2 bottle if it is in test date, but if you want a gas mix then you'll have to rent or obtain a rent free bottle or deal with a rental cylinder.
Personally I go down the rent free route and find a local supplier and pay the money..... a 20 ltr is my recommended size..
Search for :- Adams gas, SGS and Hobby Weld for you local agents