First of all, a cool tattoo is subjective. For one teenage girl, a cool tattoo would be a butterfly, but for another, the best looking tattoo she would be proud to have inked permanently to her body would be that of a cross. Thus, finding a cool tattoo may not always be the same, and should never be forced. At best, a tattoo is a personal choice. It's your message to the world about who you are.
Choosing a tattoo you think is cool must be depend on several things: design, size, price and quality. When looking for a design, the web is the best choice to go because it always being updated. There are many websites you can go to for the design or even just surfing for ideas. Remember, you don't have to limit yourself to tattoo websites, you can extend your search to animal, nature, or design sites. In fact, you can create your own cool tattoo with a little help from any one of these websites.
The size of your tattoo will depend on where you plan to put your tattoo. Obviously, since this will represent who you are (up to a point), you would want it to be visible. The question then is how visible do you want it. Would you like it openly seen, or would you prefer glimpses of it showing once in a while?
The price, on the other hand, might depend on who you choose to be your tattoo artist. If possible, choose the most expert tattoo artist in your area. For one, you can be sure to get quality work done that won't fade. Be ready to tip him if you're happy with the result.
You can even pick his brain to get some cool tattoo ideas, or how to make your design look even better. With his extensive experience and technical know-how, he will be in a position to advise you on design and color.
Finding the cool tattoo would entail making the effort and setting aside time to do the research. Take your time, and don't rush your decision. It is, after all, a permanent mark, and so you should be happy with the result for a very long time.