Seat:
Is your seat really wide enough? Can you measure the distance from the outside edge of each sit-bone, and is the seat as wide or a little wider than that? I'm wondering if you're sitting waaaay back on your seat to get the width you need.
You shouldn't be weight-bearing on the back edge of the seat. Your tailbone shouldn't be contacting the seat at all, even if you're sitting upright.
Is it your posture? Are you sitting with a rounded lower back (forcing your pelvis into a posterior tilt, which makes you weightbear on the tailbone) or is your back straight? Try straightening or even "arching" your back, does your tailbone come away from the seat?
Is it the seat position? Do you need to slide the seat back a little bit, to center it where your body wants to perch? Perhaps it is slid too far forward, but your body is still trying to sit where it wants the seat to be, so you're landing on the back edge of the seat.
TE shopping:
I get all my TE stuff through the website (upper left corner of the screen, "click to shop"). If there's a storefront somewhere, I wanna know!