Many newspapers are available for free when you board the airplane. Don't be a fool and pay before you board, but find out before you even get on the plane.
Many airlines offer free newspapers to their customers. It depends what time of day you fly, but if you get a flight in the morning it's likely you be given a free paper. Each airport will be likely to have different deals with newspapers, so unless you are travelling first class, don't expect you favorite daily to be available. If you want to ensure your newspaper is available you need to check with the airline before flying. When flying a common route often you'll be able to learn which newspaper will greet you when taking regular flights from Philadelphia to Montreal or leaving from the same airport on regularly scheduled flights from Philadelphia to La Romana.
But as the customers taking flights that leave in the mornings will likely take all the good newspapers, you'll need to prepare yourself for the potential that a newspaper will not be available. Bring a book or buy a newspaper yourself to ensure that you aren't disappointed or left with nothing to read when you get on the plane. Your electronic devises and inflight cannot help you here: only dead tree media is available to during departure and until you hit about 31,000 feet. Only then can you take out your gadgets and begin enjoying the inflight entertainment. So when travelling on regular flights from Phoenix to Philadelphia remember that you might need to make your own fun and won't necessarily be provided for.
What you need to remember is that you won't always get a free newspaper when you board. You should just consider yourself lucky when it does happen, rather than expect it every time you fly.