How can I locate floor joists?
The joists should be at the squeak, you just need to determine the direction of the joists. Typically, this is the shorter distance between exterior walls, unless there are interior load bearing walls used to break up the span. Some stud finders have a deep penetrating settings that might find a joist through carpet. And you can also try finding the joists in the ceiling below and measuring the distance from a common wall.