Michigan Bulb Co or Gurneys are great places online among others, or you can find a local plant nursery in your area. A local nursery is usually a better choice because you can actually select your tree and you know specifically what you are getting (if you did your research).
Finding someone willing to export to Ukraine, if that is what you have in mind could be quite a problem.
Yes, try to research on the internet, what tree you want, evergreen vs deicuous for example, and always refer to the botanical name, ie: a willow tree is a Salix. (Salix is the botanical name) However, trying to ship a seedling to Eastern Europe may have it's challenges...
You might do better shipping seeds rather than trees - but check up on your country's policies on importing foreign plants. Even if you find what you want, if it spreads horribly (like box elder...) you may not be able to import it.
