The basic traveller consists of a track, a car, turning blocks and a couple of cam cleats. There are so many different boats, it is hard to market a complete kit. Harken has in the past sold a "kit" containing the car, turning blocks, and cam cleats. The track has to ordered separately because every boat needs a different length.
There are generally 2 types of track, high profile and low profile. High profile tracks are used when it is necessary to bridge a gap or it is not possible to secure the track along the whole length and only the ends are secured. Low profile tracks are used when the track can be supported for its whole length and secured along its whole length.
The mainsheet system is sold separately. At the bottom is typically a fiddle block with a becket and a cam cleat. This is attached to the traveller car. The upper blocks can vary depending on the boat, a double block or a fiddle block are typical.
A photo of your current traveller mainsheet would be helpful. You might also contact
@Dave Groshong at the SBO store he'll know the options and can make suggestions on hardware and line.