How to buy police, fireman and paramedic cars?

Hello guys,
I was registered on MissionChief yesterday and now I have a little problem. I don't know how to buy police, fireman and paramedic cars. Can you help me guys? So I've searched everywhere in MissionChief, at the police stations, at the staging area, by trying to enter an ally, by trying to complete missions... but never worked. So, guys, can you help me by buying this cars? I am newbie here and I don't know how to do it. Thanks.