How difficult can it be to find relatives in a diferent country? (e.g. I'm from Czech Republic and I am searching for relatives in another european country or US.)
The more you know about them, the easier it will be. For instance, if you know 4 siblings, their ages, the town name, their spouses and children, then chances are good that you can find one or more. Then one leads to the next.
I received a tree from a distant cousin with some German relatives listed. It didn't take long to find a few of them using Facebook and the German Google site www.google.de
People ages 15 to 25 often have a Facebook account. Older people may be listed on business and career/resume sites. Younger people may be listed on a school soccer team, etc.
If you don't have exact names, you can find a historian or genealogist within the town by contacting the mayor's office or a local church.
I also check the public trees at Ancestry.com to see who is researching common ancestors. This has led to some foreign relatives.