Typically, a book or play will not be available for free until it is out of copyright, 50 years after the author's death. You can often purchase inexpensive used copies at amazon.com and in my experience that would be your best bet.
For this particular play, I do not see any free online texts. You can likely purchase a e-reader copy through iBooks, Nook, or whatever service you use (if you have an e-reader). You can also find the book at most book stores. If a store does not have it, they might be able to order it for you. Of course there's always Amazon, as was suggested above. You might also try a second hand book store in your area if you are looking for a less expensive copy.
The play For Colored Girls Who Have Considered Suicide When the Rainbow Is Enuf (1975) can be purchased at many on-line stores (Amazon.com, for example), but I have not been able to locate a copy of the text on-line.