Dragon shouts - how to find locations in-game?

Solution 1:

Most of the time you will find shouts during quests; couriers will deliver messages to you revealing shout locations, Sometimes the shout will be in the area the quest takes place. Just keep an eye out.

If you don't mind soiling your experience (imo) you can look up the locations for shout words online and attain them that way.

As for part two of your question, I'm pretty sure all shouts have unique words they use. So no, words are only used once.

Solution 2:

Two ways to find dragon shouts are to use your current dragon shouts in a town until a courier will come along giving you a letter which will lead you to another word wall, keep in mind, you can only have one letter at a time, you need to find the current word wall before a courier will deliver a new note.

The second way is to talk to Arngeir (who you will meet along the main storyline) and he will direct you to a word wall that you haven't found yet. Just like with the couriers, you can only have one quest from Arngeir at a time.

The two quests aren't mutually exclusive though, and you can have one of each at a time, giving you two word walls at a time to find.