Get yourself an Air Band scanner.
I used to have a Uniden Bearcat before it got pinched and before that I used to just simply use short wave on a normal radio.
Only trouble with that was that it was unreliable and I could only hear one half of the conversation, ie: ATC and not the pilots reading back.
You also need to know the frequencies of the ATC services that you want to listen to (unless they make scanners now that can seek those out for you-I don't know!).
Also, check whether it not it's legal to listen to them in Bulgaria, it may not be.