Postgres can use Ispell-compatible dictionaries in text search, but does not provide the required files.
This example uses the Canadian English dictionary, but you can try it with others as well. These steps are for Windows.
In PgAdmin, run the following SQL:
You will need to create a new text search configuration to use the dictionary.
This is covered in detail in the PostgreSQL manual, under ISpell Dictionaries.
Essentially you just