I was told by my Toyota dealer, and have heard this elsewhere as well, that it is virtually impossible to place a factory order for anything. If it's not on the dealers lot, and their distributor doesn't have the model/color/trim level you want, they will search other dealers for it, and that's it. You can wait a few months and see what become available, but there is no other way to do it.
Difficult only in that:
The lot jockeys can't do it on their own.
The sales manager doesn't want to do it, because it is likely that the sale won't go through.
The fleet manager can do it.
So first you have to get to the fleet manager, which is not hard, but I've been lied to by regular sales guys who claimed to be "fleet managers"
There are different specific cutoff dates for orders for the current model year. These dates vary by model, as they are tied to production line schedules. It is possible that the production line for some 2019 models has already stopped adding new cars. This would be likely for models that will see a facelift or remodel for 2020, such as the Highlander.
OTOH, it may be possible to order a 2019 Corolla or Camry in May or June.
Yep... Camry can be ordered until May 10. 2019 Corolla is done... 2020 can be ordered (including the hybrid), but pricing won't be out until April.
Highlander is good until August.
Even fleet will TRY to find what you want at another local dealer... it's faster, they are less likely to lose the sale (and make you happy). The first rule of sales is not to let the customer out of your sight until the deal is closed. If you go home intending to come back in 2 months to pick up your new car, they know that there is a very good chance that you'll buy something else and now they'll be stuck with your special order. New vehicle purchases are extremely impulsive for the majority of people. They may do research, but when they make up their mind, they want it to happen NOW, and this does not work in the dealer's favor if they are looking at a 2 month wait on your new car.