Avgas is available in Russia, its just not easy to find out where unless you are Russian. If you cannot fit ferry tanks assuming you have very limited range will make your requested routing almost impossible, eastern Russia has very limited access to airports in that region for non Russian registered aircraft and non Russian citizens, maybe if you advise of the type I could help you more as I fly regulary in Russia. Otherwise to miss Russia you would have to fly down through the Aleutian Isles then directly to Japan assuming you can get permission to operate in to some of the US airbases.