Recommended translation: Timaeus, trans. Peter Kalkavage (Focus, 2001).

Excerpt:

Socrates
One, two, three,—but where, my dear Timaeus, is the fourth of our guests of yesterday, our hosts of today?

Timaeus
Some sickness has befallen him, Socrates; for he would never have stayed away from our gathering of his own free will.

Socrates
Then the task of filling the place of the absent one falls upon you and our friends here, does it not?

Timaeus
Undoubtedly, and we shall do our best not to come short; for indeed it would not be at all right, after the splendid hospitality we received from you yesterday, if we—that is, those who are left of us—failed to entertain you cordially in return.

Socrates
Well, then, do you remember the extent and character of the subjects which I proposed for your discussion?

Timaeus
In part we do remember them; and of what we have forgotten you are present to remind us. Or rather, if it is not a trouble, recount them again briefly from the beginning, so as to fix them more firmly in our minds.

Socrates
It shall be done. The main part of the discourse I delivered yesterday was concerned with the kind of constitution which seemed to me likely to prove the best, and the character of its citizens.

Online:
Amazon (Recommended Translation)
Project Perseus (English, Free Access)
Project Perseus (Greek, Free Access)
Project Gutenberg (English, Free Access)