Any car rental agency will be fine - but they are charging a premium for 4WD. I used my National free days, otherwise it was $225 per day plus tax for a Wrangler. One option if staying on the Kohala coast is booking one for 24hrs, picking up at the Hilton Waikoloa, but after gas and time taken for pick-up, finding and buying a take-out meal at the market, buying gas at Waikoloa prices, drop-off, etc, it almost makes more sense to just book one of the tour vans for the same or less cost unless you have a coupon or free days to burn, plus the tour comes with private stargazing, although with less sophisticated telescopes and commentary.

For the new moon stargazing, here is the link: https://hilo.hawaii.edu/maunakea/kuene/stargazing.php

However, one of the astronomers we were chatting with while acclimating, aside from regaling us with tales of his friend who would drive up to the telescopes in his 3cyl Geo Metro, did warn that the moment the reservation portal opens for the next month, the slots are gone within 30 seconds and people have the webpage loaded and ready for the very moment the slots open, so it will take some technical acumen and luck to grab a slot.