You should be able to find one right on the 2200 Lowland Track block (just next to the sign), as well as on every other quarter street. There are also around seven mailboxes in each region, so there should be one just a few streets away no matter where on Ur you are. :)
I know, what I mean is, the mailbox there was gone! And the other ones too! I haven't checked again this morning but I do know where the mailboxes *were* and they weren't there when I posted last night. -_-