The speed sensor is part of the conductor plate.
You can get one from FCPEuro ( 2003 MB SLK320 ), free shipping and lifetime warranty.
Since you are opening the tranny, you will also need a filter and approved fluid, I used Pentosin ATF 134.
There are numerous videos on YouTube on replacing the conductor plate.
Complete kit :
https://www.fcpeuro.com/products/mer...IaAkffEALw_wcB