You can try to use one from this thread.
Thank you, I used one.
For future reference, I stumbled on the information. It can be found under Solos. At the very bottom is a link for delivered and rated solos:
Click on the link to expand it and select the seller, probably works similarly for a seller to select a buyer.
This takes one to the stats page for the delivered solo.
Select Ratings and underneath one's rating images you will see a section with the discount code: