The first thing I want you to know that here in Poland for buy insurance for your vehicle, at first it must to be registered in the Official Register of Vehicles and with Polish registration. If not, I recommend that you look at the Vehicle Registration section.
After you have all the paperwork for your vehicle, buying insurance is the simplest thing in the world.
You should go to one kind of Polish establishment called “Ubezpieczenie Samochodu” where they will attend you gladly, check all the information about the registration of your car and offer you the best insurance possible because usually they work with many insurance companies at the same time.
In my case, I contracted a company called TUZ Ubezpieczenie for a year with damages to third parties (the basic one) and it was very cheap (approx. 120€ / year) thanks also to my wife because she has a 60% discount (she has registered another car for a long period of time). As in many other countries, you can register the car in your name and the insurance in the name of another person who must be Polish.
Anyway, if you have any doubts or questions, do not hesitate to contact me.