A while back I actually posted something on this topic in the Russia thread:
viewtopic.php?p=1316515#p1316515
I'll copy & paste it here for ease of access, plus some new stuff:
Russian Federation
'Пограничная служба' (Border Service)
Primarily seem to use UAZ Patriots. As a branch of the military, they use black licence plates; the number '12' represents the Border Service.
My searching seems to have found both silver and dark green liveries; I'm not sure what the difference between them is, or if one is newer than the other.
(
Silver Livery), (
Front), (
Rear)
(
Dark Green Livery)
Finland
'Rajavartiolaitos/Gränsbevakningsväsendet' (Border Guard)
Use vans, including the already in-game Volkswagen Transporter.
(
Front), (
Rear)
Estonia
'Politsei- ja Piirivalveamet' (Police and Border Guard)
From what I can gather, the Estonian border guard uses standard police vehicles, as the two services are fully integrated.
Latvia
'Valsts Robežsardze' (State Border Guard)
Use pick-ups and SUV's.
(
Fleet), (
Detail)
Bonus:
(
Full Side View 1), (
Full Side View 2)
Lithuania
'Valstybės sienos apsaugos tarnyba' (State Border Guard Service)
Use a range of vans and SUV's.
(
Volkswagen Crafters, already in-game), (
Renault Trafics), (
Toyota Land Cruiser)
Bonus:
(
Rear [in background, demonstrating that there are no special markings on the back of this specific vehicle])
(
There's text with a phone number on the rear side of this vehicle [also visible in the above], but I can't find an image with a clear view of it)
Poland
'Straż Graniczna' (Border Guard)
Mix of patrol cars and pick-ups/SUV's. Registration plates of the Border Guard begin with 'HW'.
(
Skoda Octavia Combi), (
Rear), (
Ford Ranger), (
Toyota Land Cruiser)