I've been buying tires for years from Tire Rack. I believe they still supply,among other tire outfits, like Ted said, various sizes in the Z speed rate in 15in sizes.
Your top speed allowed by the LTA at THE MAINE EVENT might be limited if your tires are of a lower speed rate, but if your car is basically unmodified , the tech people at the LTA would sticker your car up to the safe speed for the rating on the tires, irrespective of a record that might be significantly higher than the car's potential... IMHO,go Z !
I hope everybody is aware of the new regulations in place this year. Do take into consideration that the SCTA , ECTA, and LTA are very similar in most rules and regs, but they are still different, and their individual policies are tailored to conditions at their venues.
Get a Rulebook for every specific "TA" or track you might race at, and try your best to be prepared for every eventuality. It is possible to construct a vehicle that passes Tech at all venues, and better yet, makes you the driver confident that you won't look back after a bad case scenario and say "woulda, coulda ,Shoulda !"
Keep your Maine Event, Uneventful .....
Bob W