Insurance for a foreign vehicle in USA
 

European insurance for a vehicle is not valid outside the "green card area" which includes mostly Europe and Russia.

Where can I get a proper insurance when entering US with my Finnish motorcycle next spring?

I think you can get insurance for foreign registered vehicles here.

https://www.progressive.com/

Check Progressive & Geico.

Also plenty of info about insurance can be found here: Overland United States of America Travel - WikiOverland

If you are also hoping to get full comprehensive insurance, you might be able to get it in the US, but elsewhere it may be only mandatory liability insurance and you will be self-insuring your own vehicle.