Why buy a new spray bottle for your plants? It is far more environmentally friendly to recycle an old one. And a spray bottle that contained some sort of cleaning product (Windex, Mr. Clean, etc.) is easy to recycle for horticultural use.
Simply rinse the bottle, fill it with fresh water, spray a few times to rinse out the pump mechanism, then rinse, refill with clear water and spray a second time. Now pour the diluted product of your choice (clear water, insecticidal soap, rubbing alcohol, fungicide, herbicide, etc.) into the bottle and you’re off to the races.
One Product Per Bottle
Ideally you should dedicate a separate spray bottle to each product. The “insecticidal soap” bottle, for example, should only be used for insecticidal soap and nothing else. Certainly, never fill a bottle that ever contained a herbicide with any other product whatsoever: even a trace of herbicide could seriously damage the plants it is sprayed on.
To avoid errors, simply write the name of the product it contains on the bottle using an indelible pen. That way there will no possibility of error!