I guess the first thing to do is find an answer to your second question !
There are 3 lists to choose from, I will list them in order of popularity
EasyList
Fanboy
Adversity
Maybe the best way to go about choosing which list to supplement is to try each of them out for a while and see which does the best job of taking care of ads on Latvian with the least amounts of False positives.
After you have chosen a list to supplement it's just a matter of adding Latvian filters to take care of any ads that aren't already blocked.