If you are a self-traveler you probably will try to buy a sim card since you arrive. Supposing you failed to find the store at the airport, go to the city by aeroexpress where you definitely find it.
Below you'll see the list of the stations you may arrive at by aeroexpress and the adresses of the closest stores of top-3 Russian mobile operators.
Domodedovo airport -> Paveletsky railway station- MTS & Beeline - Paveletsky railway station, Paveletskaya square, 1
- MTS & MegaFon - Kozhevnicheskaya street, 5 building 1
- MegaFon - Valovaya street, 2/44 building 1
Sheremetyevo airport -> Belorussky railway station- MTS - Belorussky railway station, Tverskaya Zastava square, 7 building 1
- MTS & Beeline - Belorussky railway station (the remote part), Tverskaya Zastava square, 7 building 3
- MegaFon - Valovaya street, 2/44 building 1
Vnukovo -> Kievsky railway station- MTS & Beeline - Kievsky railway station, Square of Kievsky Railway Station, 1
- MegaFon - Evropeisky shopping center, Square of Kievsky Railway Station, 2
- MegaFon & Beeline - Bryanskaya street, 2
Besides that,
MTS is going to put sim-machines in public places for everyone to buy a sim card by him/herself in 2018. So maybe you will have a chance to try it already during FIFA 2018!