Easiest/simplest would most likely be to buy the premade regulator setup outright, like GLA or Aquatek.
If you're willing to customize your own regulator, you can make a better setup for around the same price 90-120ish (that's at least how much mine cost).
Simple, least expensive outright? DIY CO2 with yeast, I don't have the links at hand, but a quick google search would provide recipes and materials!
With yeast you have to redo it every 3-4 weeks in a two liter and short term it's the cheapest, but not for the long term.
How much are you willing to spend? It'd give us a better idea of whats more suitable.