Try Duragloss #901 car wash concentrate and you will never buy another type of soap for your gun...for the price nothing will touch it. Mmmmm, cherries!
I mix ~4-6 ounces of shampoo in a gallon jug filled the rest of the way with water. Then I fill the foamgun from that. But I seem to use a much weaker shampoo concentration than a lot of people.
For OTC products, don't forget that autopaint/body supply places often have a lot of good stuff..in this case Meg's #62 an #00 shampoos.