For gameplay reasons, cash is not acquired from looting corpses or in stashes. There is however a way to take money from people by selling them items and equipment and leaving them as little money as possible. Then, by killing them, this allows you to recover all sold equipment, plus the other stalker's equipment, whereupon you can repeat the process with another stalker (although weapons sold usually become extremely damaged when you take them back, meaning you will receive less cash for them). If the player does not wish to kill a stalker, he/she can simply sell him his/her excess items (medical suppies and ammo are worth much more to other stalkers than they are in stores), which will also allow you to carry more items, as it reduces the amount of weight you are currently carrying (against the carry weight limit).
The "Ruble" is the currency of Russia, not Ukraine - the form of money in Ukraine is the Ukrainian Hryvnia.
The word "Ruble" is not written anywhere in any game - the word is always replaced by "RU".