The Scent of Search by Tyler Tate includes the following recommendation:
Place the searhbox in the top right corner of the page. Users have come to expect it in this location, so moving it anywhere else will reduce the number of users who actually find it.
György Fekete in Designing The Holy Search Box: Examples And Best Practices makes a similar recommendation:
The most convenient spot for users would be the top left or top right of every page on your website, where users could easily find it using the common F-shaped scanning pattern.