I'm glad you finally found a taste for boza. Or a boza to your taste :)
I haven't tried Harmonica, mostly because I'm getting sick and tired, and frankly downright despise that "bio" label. First, because it's stupid (what's next - geo soil and hydro water?) - "proper", "regular", "normal", "old time", "ye olde" even, would be better. I have a similar gripe with calling "a fresh" something I used to call plain old juice when I was a kid. And second, because no matter the good intentions behind trying to mass produce stuff the old way, it's getting as abused as "homemade". Write "bio" or "organic" on the label, add a small font disclaimer and up the price a couple of notches. 
Sorry, this turned into a rant. Anyway, I've seen Harmonica products in Billa on Rakovski str. and at the University, so you'll probably find them in all their stores. I think there was boza and there was definitely cheese and ayryan. And eggs, though whenever I have opened a box to check them out, they're almost always cracked. Not broken, but will break when boiled.
If you want to buy proper milk or eggs at decent prices, I can direct you to a farm just outside of Sofia, or a milk vending machine near the central railway station, stocked daily by another farm near Vratsa. I assure you, milk at the stores can't hold a candle to these two. It tastes like fond childhood memories :)