I have four Tillet T11 Seats which were specially ordered for my son from Tillet, they are unfortunately too big. I have two T11T and two T11VG, one of them has been drilled the others are brand new. These are supposedly the best seats for a Tony Kart and are hard to get.
$200 each for undrilled
$100 for drilled
I have also a Tillet T8 uncovered Small, has been drilled but is in excellent condition $85