There are three types of clusters in the game: Open Clusters, OB Associations and with a bit of goodwill also T Associations.
Open Clusters can be found by viewing one of the real world's star catalogues like NGC or Collinder (Col). Take NGC 3590, NGC 3114, NGC 3766, NGC 2367 for example. Or, looking at the Eta Carina Complex you'll find Trumpler 16 and Trumpler 14 as well as Col 228. In the Skull and Crossbones nebula you'll find Haffner 18 and 19 (if somewhat miniaturized).
OB and T Associations require a bit more search but basically any formation of massive stars (O, B, A) can be considered one of these. The most prominent you'll find in NGC 7822. However, FD is not consistent in putting these into the game. In many astronomical entries for nebulae for example you can read about their correlating more massive stars that burn the nebula away with their radiation. But when you search them they can be either omitted or located completely elsewhere. And T Tauri Associations are too hard to spot nowadays as they usually hide deep within molecular clouds that absorb their light.
You can also have alook at the maps and entries of the Galaxy Mapping Project or the First Great Expedition. Most prominent Open Clusters (and some imaginary ones) can be found there.
Hope that helps a bit.
Fly safe!
Andrew