If you are wanting to do some real quick shots, then a small 1 or 2 gallon tank that you pressurize by hand would be best. All you would need to do is figure out somewhere to mount it. If you are needing to spray a few gallons, then a tank with a 12v on demand pump is your best bet. Unless you will be doing a lot of continuous spraying, the lawnmower's battery will handle the pump just fine without draining it.
I do a little more spraying than some, so I mounted a 25 gallon sprayer to the back of my farm cart - I call it a farm cart as it no longer qualifies as a golf cart. I have a boom on the back that gives me an eight foot wide path as well as a handheld wand that gives me roughly thirty foot of reach.
