There are a lot of great resources out there for finding an apartment in North Park.
Craigslist.org has a great section for rentals and most of these are rentals by owner.
Trulia.com is also a great resource - you can select your neighborhood, what amenities you want and even put in your minimum and maximum you are willing to pay per month.
Another source I would recommend is ApartmentHomeLiving.com. It allows you to choose your state, city and then neighborhood. It has a lot of the same features as Trulia with the ability to choose amenities and prices.
Here are a few more you can try: ForRent.com, Rent.com, WalkScore.com, MyNewPlace.com, and Zillow.com.
I particularly like WalkScore.com because it gives you great information on how 'walkable' the area is, what your commute time will be and if you are near transit.
You can also use the local papers and most of them have an online presence now where you can look for apartments that way too. I have not personally used a real estate agent in the past - so I do not have a recommendation in that area.