As michael_x says, there are various screens available. In general, you have several different options:
- 7-segment displays -- without additional hardware, these become a problem as you get more digits, you need a lot more pins. There additional hardware boards to use the i2c bus, so you only need the 2 pins for i2c
- Text displays -- I've seen 16x2 and 20x4 boards being offered. Displays come either take 6 pins or so if you control them directly from the Arduino, or you use use an i2c backpack that only uses the 2 i2c pins
- Graphics displays -- The arduino really can't display much in terms of graphics due to the limited amount of memory, but you can can small displays that you can program. Usually there is a library to allow you to print text.
- Video Experimenter, TVout -- you can emit small resolution composite (analog) TV signals, either as a full screen,or overlaying an existing TV image. I suspect this is not what you want.
- There are shield versions of both the text and graphics displays that simplify the wiring, and often times present buttons to use for interactive usage. I happen to have the DPRobot 16x2 lcd shield, and the few times I've used it, it works as long as I don't use a servo (the servo library disables PWM on pins 9-10, and the lcd shield wants to do PWM on those pins).
So where to buy these screens? It depends on where you live, how much you want to pay, and how long you want to wait. You get the cheapest prices at Hong Kong internet stores, but recently, it has been running about a month from the time I order something from them, until I get it. If you live in the USA, you might want to look at US distributors such as:
Other places to look, include: