Jet streams are usually found in the :

Jet streams are narrow, concentrated bands of high-velocity westerly winds primarily located in the upper troposphere [3]. They typically flow near the tropopause, which acts as the boundary between the troposphere and the stratosphere. The altitude of these streams varies; polar jet streams generally flow at 6–9 km, while subtropical jet streams flow at 10–16 km, corresponding to the height of the tropopause at different latitudes [4]. Scientific data indicates that maximum wind speeds occur in the vicinity of the 'break' in the tropopause [3]. While the ozonosphere (ozone layer) is located within the stratosphere and the mesosphere and ionosphere are much higher atmospheric layers, jet streams are fundamentally upper-tropospheric features that follow the tropopause's pressure and temperature gradients [5].

Sources

1. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 27: Jet streams > 27.2. Permanent Jet Streams > p. 387
2. [3] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 27: Jet streams > 27. Jet streams > p. 383
3. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 27: Jet streams > Why Do Polar Jets and Subtropical Jets Flow At Different Altitudes? > p. 385
4. [4] https://forecast.weather.gov/glossary.php?word=JET
5. [5] https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC%2000-6A%20Chap%2013-15.pdf