You could request a sedative from your vet like acepromazine, although they may not dispense if they have never seen your pet before.
Another option, that can have a little bit of a sedative affect is the over-the-counter antihistamine, Benadryl.
Benadryl can be given at a dose of 1mg per pound of body weight. Keep in mind, Benadryl tablets and liquids come in different sizes. So, an approximately 25 pound dog can get a full 25mg tablet or a half of a 50mg tablet. I usually avoid the liquid Benadryl in larger dogs (you would have to give too much of it). You can give Benadryl every 8-12 hours.