The easy sources will have the most competition from other potential flippers. You might have to pay a higher price as a result, with a smaller profit margin. On the plus side, though, you can work efficiently and perform more flips than if you had to spend a long lead time to find each one.
The easiest way to find flips is to concentrate on creditor auctions and real estate owned by creditors. That means foreclosures, tax sales, IRS seizures, and similar things. The owner is motivated to sell, needs to sell quickly, and is not as price sensitive as normal people.
Buying property at, or immediately before, foreclosure is a common type of flip. You need to be careful, though, because you could be purchasing title problems, other people might have superior rights to the property and you could lose your entire investment. In addition, borrowers in many states have the right to buy their property back after a foreclosure, even if an innocent third party owns the home at that time. This is called a right of redemption.
While none of the foreclosure issues are particularly tricky, there are a lot of things to learn for this specialty. I suggest you buy a book devoted to foreclosure sales. I have two books you can read: How to Make Money on Foreclosures and The Make Money on Foreclosures Answer Book.