When hiring mercenaries, the type that are available is dependent on the wisdom check of the character seeking for such persons (see Enlistment of Hirelings). See the table below:
The table indicates the modifier to the character's wisdom in seeking the mercenary hireling sought. For example a character with a 16 wisdom makes a wisdom check in acquiring a hireling, rolling a 7. This is 9 points below the character's wisdom, so the list of hirelings that are available for hire are those with a modifier of -9 or better. The artillerist, which has a modifier of -10, means that the character would need to roll a 6 or less to have a chance of finding such a person.
Because of the difficulties in finding truly unusual or experienced mercenaries, characters are encouraged to hire agents to find such persons rather than seeking directly for them. On the other hand, easy to locate mercenaries such as light footmen or light archers may be easily found by characters who may even have a relatively low wisdom.
The table above also indicates the weapons, armor and cost of such mercenaries. At present, I have no plans to make this table adjustable for the difference between one market and another ~ but perhaps someday I might.