Expensive initially but cheaper than diy long term. It's pretty straight forward - you need;
co2 cylinder or soda stream cylinder - anywhere from $20 - $400
co2 regulator - $65 - $120 brand new
co2 grade tubing - around $10
bubble counter/check valve - $15 preferably glass
diffuser - $20ish
co2 drop checker - $18
if you do use a soda stream bottle also you will need an adapter for the regulator which I think costs around $35 - $40?
Valray from trademe sells everything there except the cylinders, these are more specialist items and my advice is to search around and see what you can get 2nd hand.
I use pressurized and diy co2 and both have their upsides, I would never use diy co2 on any tank smaller than yours now though as it just produces enough co2 for my 54ltr tank - you can supply more if you setup multiple bottles though.