You can buy a normal tile for two coins.
Central tiles can then be aquired by meeting people using Streetpass relays (much easier to complete that way).
If you have a rooted Android phone, there are a couple of topics you can find here for that.
Example:
http://gbatemp.net/threads/android-streetpass-riilay-the-homepass-method-on-your-phone.353091/
I think this one wasn't updated in a while so you probably should look for another.