Kinguin sells the cheapers CSGO cd keys by far, you can get it for as low as $8 without any special promotion.
Click the picture to buy CSGO for cheap.
A few times per year CSGO is -50% or even -75% on steam and that is a good time to buy it for your smurf, but if you are not willing to wait this is not an option.
2. Steam sales
Need a smurf or just a new CSGO account?
On this page the cheapest options for buying a new CSGO account will be listed.