I rented a car from HNL for our October trip months beforehand at a high price, but as we got closer to the trip, the price went way down. I found that it was lowest about 3 weeks before our travel dates. I'd reserve something now (assuming it can be canceled) but keep an eye out a couple of months before your trip.
I originally booked through Discount Hawaii Car Rentals (google it) but ended up getting a better deal directly through Enterprise in the end. We don't have a Costco close enough for it to be worth it to buy a membership.