Technically, disc brake mountain bike quick release rear dropouts hold 10mm in diameter axles and are 135mm wide. Most front quick rease front forks take a 9mm axle. So please double check if you do really need 9mm axle on the rear, I doubt it does, but I don't know which bike you have.
- 10 = 10mm axle diameter
- 135 = 135mm dropout width
- QR = Quick Release
Most non-boost (12x142mm) thru axle hubs can be converted to 10mm 135mm QR.
If the hub says it includes the 10x135QR, then it will fit the starndard 10mm 135 rear dropouts.
Most hubs that have interchanbles end caps are simple to change, see this short video as an example.
Here is an image with different end caps for Koozer XM490 Hubs (front and rear) which change the spacing of a hub to suit different frames and forks: