Clerics may be hired for spellcasting services as per the rules on DMG 103-104.

Magic-users and illusionists may be hired for spellcasting services, though there aren’t “magic shops” and things like that. A magic-user or illusionist must be sought out. They will almost always demand service rather than payment for their own services. In the rare case where they may be convinced to accept payment, the cost will be based on guidelines found on UA 80, and they may demand that part of the fee is paid in magic item(s).