Windscreen washer components and fish tank supplies would be a good place to start looking for parts
Water can be injected upstream as far as the air filter and high pressure is not required.
I've never done it or seen it done on a normally aspirated engine but it works well on turbo motors but they generally have much more sophisticated engine managenment systems than you'll find on an airhead.
I'm not sure how you would determine when, how much or how long to inject water on an airhead.
My bike is slow but the earth is patient.