Never just rock up to the desk at Domodedovo or Sheremetyevo without a booking, during any half-busy season you'll be left with some battered motor or a silly price. Loads of travellers sort a decent rental online and just grab the car on arrival, especially around Moscow where the aggregators show compact saloons for peanuts if you book a week ahead. You can browse what is left for your dates on this site
https://getrentacar.com/en-US/russia/moscow .