Firstly you will have to find an advocate to do this for you . Sometimes the advocates in Bulgaria are bit expensive and take more for taxes then for the real service.
The other way to register company Ltd or Sole Ltd is to search for an accountant which will do this for you maybe at the best prices . One example is this accauntant in Plovdiv which have also promo offers for some countries in EU - Company registration Bulgaria .
They can also advise you on accauntant services after that and what is good for you.
Beware of people who tells you that you have to pay many taxes for different things . This is not true . The only different thing you have to do if you are not Bulgarian is to have the Company documents in you language . This is the only specific thing in the Bulgarian trade law.
So if you want to register company cheap and fast in Bulgaria you can search someone to do this for you.