I would recommend checking with the DMV where you plan to register the vehicle before you do anything else. I used to have my own that I found online that I would use for all of my car purchases and sales.
My county DMV began requiring the use of their own specific bill of sale. My guess is they got a few too many chicken scratch on bar napkins as a bill of the sale and decided to require their own specific document that had all the relevant information they required. Subsequently, they put it online so that you can download it.
Even if your DMV doesn't require a specific one they might provide one on their county website to make your life easier.