The best thing is to CALL all the big ones (National,
Alamo,
Budget,
Avis, etal) and ASK.
It may take you 15 minutes, but minutes well spent.
I know NATIONAL specifically rents them -- for the same price as the seven-passenger at least when I search they are always the exact same price.
BUT, the "eights" do not always show up on the National "selection screen," which leads me to believe they sell out very quickly.
ALSO...they NEVER show up on National if you use KLM or a few other contract IDs (must be specifically "voided" from a number of contracts).
Just a WAG, but if National rents them and SPECIFICALLY reserves them, brandmates Alamo or Enterpise may also....but call around.