Back on topic for you.
If you currently have a donut type liner then I guess you're looking for a flat one like these:
The toyota 86 has this one
https://www.japanparts.com/parts/detail/43151
The Subaru BRZ has this one
https://www.japanparts.com/parts/detail/40672
And this
https://www.japanparts.com/parts/detail/40673
You should be able to find the equivalent in the US.
If however you mean a hard piece of board to go over the top I'm not sure where you'd find that but you could cut your own piece out.
And, off topic - our later cars came with repair kit. I got a spare when I split a tire wall and the repair kit wouldn't work. Not fun being stranded and then the hassle of leaving the car to go away and get a new tire fitted.
I also take a spare with me to the track just in case I need it to get home if anything goes wrong with a tire while on track.
EDIT: could this be what you're after
http://www.ft86club.com/forums/showthread.php?t=71895