Historically, there was no such style as Russian Imperial Stout. In a sense it is a made up style. It's a bit like saying there is a style of jam, or of biscuits called "By Royal Appointment". Historically the style most closely associated with export to the Russian Court was the Triple Stout. Single, double and triple stouts were usually brewed with the same grain bill, varying in their water to grain ratio. In some cases the beers were parti-gyled, that is the first runnings from the mash were used to brew a triple stout, and the second runnings for a single stout or porter.
If you are interested in the history etc. of these beers then Ron Pattinson's blog Shut Up About Barclay Perkins is a great resource. The following grain bill is based on Ron's material.
Choice between Barrett Burston (Aus) ale malt, or Thomas Fawcett's (UK) Maris Otter
Black malt 2.5%
Amber malt 17.5%
Brown malt 17.5%
Ale malt 62.5%
Original gravity 1100
At 240 LDK (77% mash efficiency) 420 grams per litre
At 200 LDK (65% mash efficiency) 500 grams per litre
Mash temperature 65 degrees C
Bitterness 45 to 65 IBU, suggested hops anything English (mostly). Goldings, Fuggles or Styrian Goldings for the late hops. Bittering hops could include Target, Challenger and Northdown, or any of the more neutral higher alpha hops such Magnum.
Yeast: For dried yeasts the first choice is the Mangrove Jack New World Strong Ale, Nottingham as a good second choice. Both of these are fairly attenuative yeasts. Safale US-05 would do at a pinch. Using Safale S-04 would give a slightly sweeter finish, it may pay to drop the mash temperature a degree.
With liquid yeasts there is quite a choice. Wyeast 1728, Scottish Ale, comes up well in this style. Any other strains, whether Wyeast or White Labs, recommended for the style will be good, although with the less attenuative strains it may pay to mash a degree cooler.