Are there any prepaid cards that can be used in any transaction online in Russia?
If so, where do I buy them?
I didn't know anything about prepaid cards before this question, but it looks like the prepaid cards can be a
Mastercard type, which both are being used in Russia widely.
So, if you do bring some of such prepaid cards in Russia, you probably be good with payments made in places accepting two mentioned payment systems. As all over the world, you can see some stickers on entrance or near the cashier about what types of cards are accepted.
In general people in Russia do not understand the difference between those system, and call them just a "cards" (карточки, kartochkye). Usually some big bank do provide the service for a card machine, and they do not have problems with either system, but it really depends on city, place and time you try to do a purchase.
As for online transactions, the payment agent usually do not support cards from other countries rather than Russia, so you need to visit a bank to buy such card. For example, Russian biggest bank "Sberbank" offers such cards for sale in Moscow and Saint Petersburg:
You can purchase a prepaid card in Euroset outlets located at: 32 Zolotorozhsky Val, Moscow, and 36/40 Sredniy Vasilyevskiy Ostrov Avenue, Vasilyevskiy Island, Saint Petersburg
Update: The prepaid cards you're talking about aren't much popular in Russia yet, but you still can try to investigate this: Russian Standard Bank' cards (also they are related with this website, but I can't find any information about it's real usage experience); GazProm Bank' cards (you can buy them in bank's branches); and, as expected, SberBank is providing some virtual cards too, and it looks like they can be bought online.