My advice is if you can't afford to pay for it, then forget about it. The plants will still grow. No point trying to mix something up which could be pottentially harmful and kill your fish and even you just to save some money. If it is as simple as mixing some chemicals together then it won't be that expensive to buy.
Injected co2 is not that expensive, only the innitial set up.
Just my oppinion,
ron