Where can I buy a second hand transmission for a 17.5 hp yardworks lawn tractor

Start visiting small engine repair shops or look on Ebay and Craigslist.

Hi,
If you provide the model number, I will attempt to get you a part number, and maybe a parts breakdown. It is made by MTD. If it has forward, neutral and reverse in the transaxle, that will reduce it down to left or right shift.
Peace,
Clarke