spotting the military aircrafts in Sofia is hard, there is no good spotting position, but if you take a walk on the northern side of the airport maybe you will find some place to take nice shots from.
There will be An open day at Graf Ignatievo on the first of june, as well a military excercise with bulgarian and USAF planes (F15), but it won't be accessable for the public, unless you are part of aviation magazine etc.
If you need a guide around SOF let us know where you come from and when exactly
Looking forward to hearing from you!
P.S. Do you have any of your pictures uploaded somewhere?