A pillow that will NEVER go flat is a natural LATEX foam pillow.
I have them on every bed and would never buy anything else.
I have had the best luck finding them online. There are firm and soft styles. Be aware that firm is pretty firm in natural latex. The pillows are more expensive that regular foam. They are also hypoallergenic.
I bought the last pillows from "Foam Sweet Foam", but there are other brands. They come in really small packages.... you will be sure they sent the wrong thing! If you have a young child, be sure to let them watch while you let the pillow re-inflate! It's quite fun!