I search around the thrift stores and pickup old GPS units, typically the small Garmin Nuvi units, don't worry about the map, it does not need to be up to date. I typically pay 5-15 bucks for these, and now have several. They are absolutely accurate, far more accurate than a speedo
I've also got a small Garmin "Etrex" batterypowered handheld unit which I use in the boat
