Another question here mentioned that there are more dragons in Middle-earth than just Smaug.
Why did Sauron not search them out, if Gandalf thought that Smaug was important enough to create a quest to get rid of the dragon, because of the danger that he posed?
Not all dragons are created equally
In particular Smaug was the mightiest dragon of his time, which Tolkien confirms in Appendix A to Lord of the Rings (my emphasis):
So the rumour of the wealth of Erebor spread abroad and reached the ears of the dragons, and at last Smaug the Golden, greatest of the dragons of his day, arose and without warning came against King Thrór...
The other surviving dragons are therefore lesser dragons, and not as useful to Sauron as Smaug would have been.
To what degree was Smaug more powerful than them? We don't know - Tolkien doesn't say.
I don't think a firm answer can be given, but it's debatable to what degree Sauron (his powers of command having been reduced by the lack of the Ring) could have commanded Smaug or any other dragons. Perhaps he might have enticed Smaug to strike with a promise of spoils, or engineered events such that Smaug would be drawn into the conflict by local fighting disturbing his region, but on the whole, dragons may not have been very reliably commandable; even Melkor had some difficulty with Glaurung revealing himself prematurely (see https://www.quora.com/Why-didnt-Sauron-use-dragons-in-the-war-against-Middle-earth). Drawing other dragons into the battles in LotR in the might have required overcoming their reluctance to leave their own distant hoards undefended.
For that matter, AFAIK, we don't know for certain that Sauron didn't find some use for some dragons elsewhere. Given some of the armies (Easterlings and Haradrim) that were drawn to the fights in LotR, we're given hints of Sauron's influence or machinations in far lands. Recall Sam reflecting regarding a dead Southron: "He wondered what the man's name was and where he came from; and if he was really evil of heart, or what lies or threats had led him on the long march from his home; and if he would not really rather have stayed there in peace..." This is just speculation, of course, but it may be useful to remember that there were clearly things going on the world besides the central battles explicitly described.
A possible (admittedly speculative) explanation: they may have been thousands of miles away from the action. There is a lot that we don't know about Middle-Earth, but we do know that it extended far to the east and south of the regions shown on Tolkien's maps. It may not have been practical (or worthwhile) to lure whatever lesser dragons were lurking in these regions to the battlefields in the north-west of the continent.
I agree with the points made by Victim of Circumstance, Jacob C. and Ian Thompson in their answers. I would like to add the following points.
He didn't have the time or resources to try every strategy that we can dream up
Powerful as he was, Sauron doesn't have an infinite amount of time or resources. The search for dragons would have taken resources away from other efforts, such as recruiting the Haradrim and the Easterlings, that he might have thought more likely to bear fruit.
He didn't think he needed additional help to win battles
Also, according to his wisdom, Sauron knew that the forces he had were enough to ensure his victory. While using the palantír, Denethor is convinced of Sauron's certain victory.
‘My lords,’ said Gandalf, ‘listen to the words of the Steward of Gondor before he died: You may triumph on the fields of the Pelennor for a day, but against the Power that has now arisen there is no victory.
The Lord of the Rings Book Five, Chapter 9: The Last Debate
Page 878 (Single volume 50th Anniversary Edition)
Sauron comes to realise that the Ring is all that's important
Gandalf explains this to Aragorn, Legolas and Gimli:
The Enemy, of course, has long known that the Ring is abroad, and that it is borne by a hobbit. He knows now the number of our Company that set out from Rivendell, and the kind of each of us. But he does not yet perceive our purpose clearly. He supposes that we were all going to Minas Tirith; for that is what he would himself have done in our place. And according to his wisdom it would have been a heavy stroke against his power. Indeed he is in great fear, not knowing what mighty one may suddenly appear, wielding the Ring, and assailing him with war, seeking to cast him down and take his place. That we should wish to cast him down and have no one in his place is not a thought that occurs to his mind. That we should try to destroy the Ring itself has not yet entered into his darkest dream. In which no doubt you will see our good fortune and our hope.
The Lord of the Rings Book Three, Chapter 5: The White Rider
Pages 496-7 (Single volume 50th Anniversary Edition)
