We use http://eisoukr.guaranteefund.org/
(if the site loads in Bulgarian, click on the Union Jack flag in the top right)
You can enter the license plate and it will give you insurance info, but there is a trick to looking up previous license plates.
You must go to "Frontier insurance check" (the list on the right). There you can enter the license plate. Almost every time it will tell you this car doesn't have Frontier insurance and it will give you the car's VIN. Copy it.
Now, go back to the main page ( "Motor third party liability insurance check" ) and search only the VIN. It will provide you with a full list of insurance logs for this car, including the old plates.
Unfortunately, this whole check doesn't work if you are searching an old plate of a vehicle, which currently has a different (unknown) Bulgarian plate, as it won't give you the VIN.