I have seen it at WalMart in the past, but that was some time ago. It is usually called Fogging Oil or Engine Store or something like that. I usually pick mine up at the local BassPro simply because it is located not too far out of the way on the way to my parents house. I got a can a few years ago that had a hose and a schrader valve connection (like a tire valve) on it. There is a schrader valve on the intake on our Evinrude that you can hook the can to and fog all 6 cylinders at the same time. I just keep the hose and put it on a new can each year. When I put the boat up for the winter and start spraying the fogging oil in the smoke gets so thick in the backyard you can't see the neighbor's house! After I shut it down I put the regular aerosol cap and straw back on the can and pull the plugs and put a shot in each cylinder. It is kind of hard to start in the spring but the motor is well protected.
Mine is a 1993 175 Ocean Pro (Loop Charged or "looper")so it may be different from yours. On mine it is covered with a red cap that looks like a valve stem cap on a tire except it is pointed on the end. On mine you unscrew the cap and the schrader valve in underneath.