Here is what you need along with some links to ebay items.
A co2 source, usualy a fire extinguisher. http
/www.ebay.co.uk/sch/i.html?_from=R40&_trksid=p4712.m570.l1313&_nkw=co2+fire+extinguisher&_sacat=See-All-Categories
A co2 gas regulator http
/www.ebay.co.uk/itm/SWP-Side-Entry-CO2-Gas-Regulator-2-Gauge-Welder-Welding-Pub-Bottle-Beer-/140776182405?pt=LH_DefaultDomain_3&hash=item20c6ea1285
Some co2 proof tubing. http
/www.ebay.co.uk/itm/Rhinox-CO2-Proof-Tubing-Live-Aquarium-Plant-Diffuser-SI-/250445502160?pt=UK_Pet_Supplies_Fish&hash=item3a4fb716d0
A non return valve. http
/www.ebay.co.uk/itm/2x-Aquarium-NON-RETURN-Check-VALVE-for-air-pump-airline-/270668494505?pt=UK_Pet_Supplies_Fish&hash=item3f051976a9
A drop checker & reagent to go in it. http
/www.ebay.co.uk/itm/CO2-Drop-Checker-with-PH-solution-check-co2-level-UK-SELLER-/221047964284?pt=UK_Pet_Supplies_Fish&hash=item33777c427c
A bubble counter (I dont use one but a lot of people do) http
/www.ebay.co.uk/itm/FLUVAL-Co2-BUBBLE-COUNTER-88g-/310409015219?pt=UK_Pet_Supplies_Fish&hash=item4845d1afb3
A co2 diffuser http
/www.ebay.co.uk/itm/Glass-Co2-Nano-Diffuser-Aquarium-Fertilizer-UK-Aquatic-Plants-Like-Dennerle-JBL-/230796352025?pt=UK_Pet_Supplies_Fish&hash=item35bc88da19
That is for the basic set up, but it will mean that you will have to turn it on & off by hand every day.
If you want that bit automated you will need a solanoid valve. http
/www.ebay.co.uk/itm/CO2-Magnetic-Solenoid-Valve-for-Aquariums-0-0-8mPa-/350481860748?pt=UK_Pet_Supplies_Fish&hash=item519a58c88c
Ther you have it, add up the costs to give you an idea of inital outlay, but once purchased all you will need to do is replace your co2 supply when it runs out.
A lot of those listings are from abroad, but I did a very basic search. Take your time there are bargains to be had.
Tom