[emtor]

The seller won't help you if he/she's hard to reach.

Open the hood and locate the gear-box. Somewhere on it you'll find a sticker with a barcode and some letters and numbers (if it's still there).
It's most likely located under the air filter box.
The gear-box type will be printed on the sticker.
In your country there surely is a website that car salvage firms list their car parts.
Alternatively trace down as many of these firms as you can and call them.
A used box isn't nearly as expensive as a new one.