You can download an electronic dictionary for free from
http://www.thediction.comBut be careful. There are things about Bulgarian that you can't learn from a dictionary. Most words have multiple meanings and it is very easy to use the wrong word for a given situation. Also, the way a word is pronounced, including where the accent falls in the word, doesn't come through in a dictionary. Without having someone to teach you how to use those words in the dictionary, you could be creating some bad language habits that you'll have to un-learn later.
Having said that, I applaud your desire to get started. My advice, however, would be to place your precious preparation time reading the scriptures, going on splits with your local missionaries (don't they call them exchanges or some such now?) and generally preparing yourself spiritually. That will be of the most worth to you. I'm not saying the language isn't important...on the contrary. I'm just saying you might want to wait to till you get to the MTC to hammer it. If you do want to learn something, I recommend simply starting with the alphabet.
http://www.phespirit.info/alphabet/bulgarian.htmMay the Lord bless you in your preparations. You are coming to a wonderful country and will face many unique challenges. If your heart is in the right place, your mind will follow.