All visitors come from civs (and roving bands of vagrants, it seems. I have had two visitors who didn't belong to any civ, but rather vagrant groups based in camps), and are historic persons. If all dwarves are extinct you can't get dwarven visitors, and if your civ is dead there is a very low probability of there actually existing any non necro dwarves in the world, and if they do (I'm not sure it's possible), they're probably members of the (usually goblin) civ that conquered your site and killed off your civ. If you embark sufficiently far away from them that another goblin civ is closer to you, you should theoretically be able to receive a dwarven visitor, but getting one of the 40 dwarves rather than one of the 40000 goblins are slim odds.
Thus, if you want dwarves you should expect to grow them yourself, making aggressive use of pre honeymoon suites and baby making encouragements (it took my dorfs 20 years for the first couple to marry and another 10 for the second one. 10 additional years provided two non marrying lover couples. Some pre honeymoon work raised the number of baby producing couples to 8 [that includes couples formed from the offspring of the first two couples]).
Provided the human and elven civs are sufficiently healthy to produce visitors, you can also get humans, elves, and goblins to get down to fortress population production activities. Animal peoples should work as well, provided you can get a marrying/married couple.