I bought this
Gilmour brand foam gun off Amazon and could not be happier with it. No need for a power washer with this thing. Just fill it up a little bit with your favorite car shampoo (I use Sonax shampoo) and connect your standard garden hose. Nice and efficient/quick coverage with no waste. I probably use 1/3rd the available volume of the foam gun's tank, so it's not like you have to constantly refill it and use a ton of product. IMHO, for the price, there's no reason not to own one and give your car a quick foam bath and rinse before the actual hand wash to get the larger grit off the car first. As for construction quality of this particular foam gun, it is excellent and I expect it to last a long time provided I don't bash it against the concrete repeatedly or run it over.
Good luck!