Owl Post is located in the owlery which is next to the Hogwarts Express. It's tucked in the back corner so you might not see if from the outside depends on the crowd level. Also you can get to it through Ollivanders/Dervish and Bangs shops.
They sell fancy stamps inside the stores and at the post. They are real stamps but are sealed up and look like collector series form the USPS. You can but HP postcards in the shops or at the post and send them to anywhere you want. They mark them with a hosmeade stamp and send them out. It is a USPS authentic service. In June it wasn't ever that long of a wait...maybe 10-30 minutes and it's under the cover of the owlery so it's shady.
The postcards and some of the HP merchandise is sold in the store at the entrance of IOA if you want to buy them before you enter HP land.