Generally public parking in downtown Kailua-Kona is free but you do need to know where to look. There are some paid lots but you should never need to park there. All he parking lots mentioned below are the best to park in to easily walk into town.
1. One free lot is adjacent to the Kona Public Library and is the same lot where there is a outdoor farmers market. Right next to it is a smaller paid lot so do not go there!
2. The next best is the parking lot across from the Shell station at this address:
75-5700 Kuakini Hwy
Kailua Kona, HI 96740
You can park there overnight but you can park there all day for free.
3. The next best is in Coconut Grove where Outback Steakhouse. Parking is free there all day long.