Because this is such an important and famous text, there are a zillion places you can read it. Here's a sampling:

• For the PDF of the version published in Dr. King's "Why We Can't Wait," try the King Encyclopedia.
• For a digital scan of one of the first drafts compiled from the pieces retrieved by his lawyers, plus a recording of Dr. King reading an embellished draft (yes, please), use the King Institute.
• And here's the version that appeared in August of 1963 in The Atlantic Monthly.