You can buy some dolls and ornaments at the Slateport market, desks and chairs at Fortree City, and a whole lot of different objects like mats, posters, cushions, etc. at the Lilycove Department Store on floor 5F.
In the west part of Slateport City
They're used for purchasing Secret Base locations and decorations.
Even though I don't play Pokemon Diamond, I know you buy it on the fourth floor of Velistone Department. They will then transfer to your PC in your Secret Base.
Secret bases are hidden around hoenn its a hideout for you, all you have to do is find a big bush or a tree with vines or an indent in the wall and you use the TM secret power on it and a hole will appear or a vine inside is an area where you can use decorations to make your own secret hideout, if you mix records with friends they can battle you in your secret base and you can do the same if they did that.
All decorations can be found in stores throughout the game, you just have to find the correct store.
The presidents secret base is in U.S.A the 'pentagon'.
Well first you need the tm secret power. You can get decorations from the shipyard market place in stateport city once you have this. Teach secret power to a pokemon and use it on an indent in a wall or a tree or a bush that you can use secret power on. Then go inside and go on the pc thats in there. Go to decorate and press on one of your decorations and then place it where you want(: hope this helps.
You cannot fight people in your secret base.
you need a Pokemon that knows secret power use that move to make your secret base
on sapphire/emerald this one boy gives you the move secret power. use it to make a secret base!
you cant cover the holes in your secret base.
To find your Secret Base, talk to the short guy in a house in western Mosdeep City. He will mention a base on a certain route. Go there, and find your Secret Base on that same route!
You cannot make a secret base in Pokemon soul silver.