Normally you can get a decent price with a car buying service like Costco offers, although people who are good at negotiating can usually get at least the same deal. However, the Palisade is in short supply, especially in higher trim levels. Chevy Yukon's are almost imposable to get, so you are not going to get a really good discount on any vehicles in short supply, especially an SUV, just because you are using a buying service. If you go down in size to something like the Santa Fe (or competing brands) the supply situation is a little better, but the large SUV's are definitely flying off the lots.
If a dealer does not have the trim, color, and options you want, they can put is a request to the Hyundai distributor for getting that in their next allocation, but it just depends on supply and demand and it can take a long time since the Palisade is made in Korea. Dealers list their inventory on their websites, and on some sites I think you can see multiple dealer inventories all at once, and sometimes these include vehicles that are on the way to a dealer, but not yet arrived. So you can do some of the leg work yourself in finding what you want. Sometimes dealers will trade with other dealers.
Just remember that like real estate, everything is negotiable in the auto business and is based on supply versus demand, and right now dealers have the upper hand due to very high demand and short supply (particularly due to the computer chip shortages plaguing automakers and others).
I tried to buy a dishwasher recently at Home Depot, and they had very few to chose from, and prices were sky high. As people are getting the stimulus money, things are getting worse.