I use the gallon liquid starch (whatever brand the store has) and mix it 50/50 with water.
The best bottle I've found I get at Sally Beauty Supply. It is actually for hair spray and misting hair. It is a fine mist. I haven't had problems with it clogging but if I did I would just soak the spray part in a little warm water and it would free the clog. I also use this type sprayer for Best Press to make it go further and get a more even coverage.
As far a starching and coverage I spray my fabric and then roll it up and let it rest in a plastic bag - like we did with sprinkleing clothes back in the day. This way the starch has a chance to get absorbed into the fabric. No goo on your iron and nicer finished product. I have a plastic dish pan that I put the fabric in to spray it to keep the mess contained.