There is a company out there that makes a foam marker system for walk behind fertilizer spreaders. I have 3 of them. They were relatively cheap at $150. As far as components go, it has a low volume air pump, switch, battery, plastic jar, plastic hoses, and a foam nozzle. Get the 12v model, and you can use the smallest motorcycle battery to run it and save on throwaway batteries. I believe the name of the company is Richway or some variation of that. You may want to try this question on Lawnsite or lawncare and landscaping Forums.