You can get one firearm at Veluca University for testing purposes.
As for faction choices, it depends how you like playing. If you like mobile, aggressive warfare the Khergit are a good choice, or if your prefer a solid & disciplined combined arms force Alania or Gorinia are good choices. If anti-cavalry is your thing, some Geroian pikemen would be good for you. Aurothia is also intereting, with excellent skirmishers and medium infantry and cavalry that are an excellent compromise between heavy knights and fast horse-archery forces.