Where would I go to get a direct mail list for a certain area that I'd like to reach out to? City court house?
Many counties have online GIS databases, but you will have to scrape the info manually. A better alternative is to buy a list from a list broker such as Listsource.com or Melissadata.com. You can choose the criteria you want and buy the list.
Agree with @Chris Eaker
There are plenty of sources. A quick search will give you them all relatively quickly.
I use a combination of sources. My advice is to do a few days of research on the basics first, your question leads me to believe you have a little more to grasp.
Then...go for it!
I always suggest working with somebody to make sure you get the exact list you want and don't make a few thousand dollar mistake on something silly...
Depending on your area...I would also suggest sending out a mailing that is repeatable and defined.
A mailing list of 300 postcards will not be ideal for you right now. I'd suggest a minimum of 1,000 cards all from the same "criteria". Do not mix different lists at once during this stage.
Hope this helps a little.