4340 (SAE J404, ASTM A29, SAE AMS 6414/6415) is a Ni-Cr-Mo alloy steel that is similar, but with Ni only in the range of 1.65-2.00. It can be quenched and tempered to the same range as Condition X. It is widely available from various distributors and service centers. 300M and 35NCD16 are some other Ni-Cr-Mo steel alloys that you may want to consider. Here are some datasheets from Cartech/Latrobe Steel: