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.