D Visa for USA Citizens

This document links to the two pieces of legislation that cover getting a D-visa... http://www.mfa.bg/uploads/files/VISA%20(TYPE%20D%20VISA)(1).pdf

Warning- the two documents are long and confusing. I found the Bulgarian embassy was great at responding to me when I emailed them about getting a visa for my parents and letting me know what visa they would be eligible for and what documentation they would need. i would definitely shoot them an email before your embassy appointment to make sure you have all of the appropriate paperwork, because I have read other stories of people's visas being significantly delayed because they did not have the right paperwork.

From what I read, if you want to enter on the basis of being a freelancer, you will likely need a work permit issued to you from the department of labor approving you to be a freelancer before you can get your visa. It was a PITA to register as a freelancer in Bulgaria, not sure the process outside of Bulgaria though. But they don't issue a visa just because you want to live here- you need a valid reason (work, a functioning company that employs Bulgarians, being a family member of a Bulgarian, being retired, or investing a large sum of money are the standard reasons), so I am guessing the freelancer work permit will be necessary.

Additionally:
Your apostillized FBI fingerprints (apostilization can take a month if you do it by mail, so do it well in advance).
Proof of means of subsistence... usually a certified statement from your bank stating your balance.
Proof of accommodation. Not sure what they want from freelancers. I just used my in-law's address.
Proof of insurance. But in most cases you can hold off on buying medical insurance and buy it when you land- it is MUCH cheaper. (Like, I was looking up insurance online and it was $500-$600 a year for expatriation insurance. If you buy it in Bulgaria, it is usually less than 200 BGN a year.

Good luck.