This usually requires a motivated owner who owns their property free and clear.
Don't expect a bargain, if the seller is willing to take back a mortgage from you, the price will probably be at or close to full market price.
You should expect to make a down payment, typically of 3-5% of the value of the home. The mortgage will probably not be for 30 years either, although it may amortize over 30 years. Usually someone taking back seller financing will want to be paid off within 10 years or less.
You can get more information on seller financing on the web site of mortgage-investments.com.