These structures can be represented by the wooden pieces from the game Entdecker.
A prospector can be placed in any hex, and collects one raw material card when that hex's number comes up, at which point the prospector is removed from the board. Though far less prosperous than a settlement, it has the advantage of being able to go anywhere. A prospector can be built by turning in any two resource cards, and any number of prospectors can coexist with other prospectors or structures in a hex.
Use the scout pieces from Entdecker to represent prospectors.
A trading post, like a prospector, is placed in the center of a hex, and provides one raw material card when that hex's number come up. It represents a frontier trading center, and can only be placed in a hex that has no settlements on any vertexes. It may be placed on a hex that already has a trading post.
As soon as a settlement is built on a hex containing a trading post, the trading post must be removed. At some point in the game, it may not be possible to build a trading post any more.
Use the score markers from Entdecker to represent each player's trading post. A trading post costs one wood, one wheat and one sheep.
A fort protects a hex from the bandit. It may be placed in any hex on which a player has a settlement (unless that hex already has a fort.)
The bandit cannot be placed in a hex containing a fort. If a bandit is in a hex in which a fort is being built, the bandit is immediately expelled to any empty desert.
Use the fort pieces from Entdecker to represent forts. A fort costs one wood, two brick, a sheep and an ore.
A capital is created by upgrading a city or metropolis. It is worth 6 points, but collects two raw material cards like a city. A metropolis that is converted to a capital produces fewer resources and loses its ability to open up adjacent vertexes. Any settlements that were built when the new capital was a metropolis remain, but no new ones may be created in vertexes adjacent to a capital (there needs to be space for all the monuments). This applies even if there is another metropolis nearby, i.e. no new settlement can ever be built adjacent to a capital, despite the rule for the metropolis (the people of Catan love their monuments.)
Use the capital piece from Entdecker. A capital costs three wheat, three sheep and three ore.
This page by Steve Barrera 2001-2013