If you are unsure or wary of doing DIY co2, the hagen kit is the next best thing. I've had one for just over a month, and the difference to the plants is amazing. Better growth and deeper colours. It comes with three lots of the yeast mixture (upto 3 months worth) to get you going, and after that, you can do a search for yeast recipes on this forum. This will keep the cost down to about £6 for a year!
Hope this helps!