I am not a professional and I know this can be done with a small shovel, but I am going to be assisting in maintaining the lawn care of several properties. I would prefer to make make job a little easier. Are there inexpensive probes under $20? I found some at lesco for 17" $27.58 and 21" $32.32. What size do I need and can they be purchased at local hardware stores/nursery? Thanks for the help!!