Israel should buy the S-300 from Russia for a premium (say double or even triple what Iran is prepared to pay for it) on the condition that Russia does not sell it to Iran. Russia would still have a contract (and a far more lucrative one at that) Israel would obtain a state of the art air defence system (which it desparately needs in light of the tiny airpace it defends), and further improve its blossoming military and economic relationship with Russia. Iran on the otherhand would be further isolated as one of its only friends in the international community would have abondoned it. Everybody wins (except Iran).
EU's Mogherini in Tehran to talk nuclear deal, regional issues (Reuters)
from the article: Netanyahu to Hamas: Israel's offer for Shalit is final