These days, the iPhones can be purchased in a legally unlocked form. Therefore you can avoid the need for operator tie-in if you are willing to pay the upfront cost of the phone (rather than being tied to an operator and benefiting from amortised phone cost).
So all you then need to worry about is getting hold of a nano-sim from the Greek and Bulgarian operators. Hopefully they should be happy to oblige once they get some into stock.
I would caution agains the DIY approach advocated by some of chopping up your bigger SIM card to fit. Much better (and safer) to get hold of a correctly sized SIM in the first place.