Visa letter can be obtained in following ways:-
1)Contact the Hotel you have shortlisted and are planning to book.They would ask for your credit card details to give you the letter in case and would keep some amount on hold (in case you dont turn up at that hotel).If you actually do turn up,it basically becomes free.
2)Contact the sightseeing tour company if you have booked any sightseeing tours or day trips.
3)Search on google.There are several websites that charge a fee of 20 or 30 USD and provide the invitation letter. I would suggest to go with option no.1 and thats exactly what i did.One thing to keep in mind is if you are travelling multiple cities in russia,you need invite letter for all the cities (so again request the hotel or sightseeing companies).Let me know if you have further queries. You can go thru my travel story if you need further information on Russia tour.