All hostel, hotel and motel are places where you can sleep for a night, or longer. Boarding houses do not, as a rule, accept people for just a night or even a week.
In a hotel or a motel you will have a room to yourself, or at least to the people in your party.
In a hostel you can sometimes get rooms like that to, but mostly you will share a room with several others, you paying for just one bed, them being strangers who also just paid for one bed. Often the beds are stacked, one above an other, but that is not always the case.
An other difference is that a hostel almost always has a kitchen the guests can use, and other common rooms that are in use.
Motels are mostly for people traveling by car, are on or near motorways, can be hotels but for the location, or can have little apartments with a kitchen and a living and a separate bedroom or even several bedrooms for a price of just a common hotel room.
Hotels also have apartment and suites with a living and a bedroom, but than you pay big bucks.
Boarding houses can be hostel style or hotel style, or even have living/bedroom combinations, depending on location, target user group and price.
The easiest way to get hold of addresses is searching the internet, just google on hostel, hotel or motel and the name of the town or area and they will come up.
Other ways is to buy a guidebook, Lonely Planet and Lets Go (and others) list the hostels first, and hotels and so on, other guidebook series might list hotels or motels first and hostels only when they are not available.
Hostels are often part of a national or even international organisation, Hosteling International is the best known world wide:
http://www.hihostels.com/
Through them you can come in contact with the national organisation for your country, where you can become a member. As a member you get a hostel list for your area, sometimes the international handbook, which (used to) has two editions each year. (I have not used those handbooks for a while, so this information might be outdated.)
Other hostels sometimes work in national or continent wide organisations too, just check any indication they do.
But most people use a site like Hostelworld:
http://www.hostelworld.com/altindex.php
Or hostels.com:
http://www.hostels.com/
Or one of the many other booking sites.
I often use hostels, used to sleep in the dorms, but I am going for private rooms or cheap hotels more now.
I travel often in Europe and farther from home.