I installed these seats I found on Ebay.
They look a bit boy racerish but they were cheaper than having the originals recovered. I had an upholstry shop recover the backseat which cost me more than I paid for the car!
What is odd about these seats is they feel like they sit higher than the originals but when compared to the passenger seat (with both seats inside the car), they appeared to be the same height off the floor.
I made my own adapter. I just took a piece of aluminum and this bolts to the floor mounts and the seat bolts to it.
I installed some seats out of an 87 Honda CRX in my Spider. They fit perfect. I ended up ordering some seat covers rather than having they recovered.