What defines “two sides of a river” nowadays (in regard to lifnei evar) where we could get all over the world within days?
Answer:
In general, where there is a significant difference between immediate access to the issur and between obtaining it with difficulty, many maintain that this is considered two sides of the river.
Best wishes.
Sources:
A possible simple reading of the Gemara is that if the potential transgressor can commit the sin without assistance, even if this requires considerable effort or expenditure on his part, the prohibition of lifnei iver will not apply. This understanding appears to emerge from Rashi, who makes no qualifications to the limitation of “two sides of the river.”
The Meiri, however, writes that if the forbidden food cannot be accessed by the transgressor “without great effort,” it is forbidden to make it easily accessible to him.
The Kesav Sofer (Yoreh De’ah 83) goes a step further, and writes lifnei iver does not apply only if the given object is near and accessible to the potential violator. If he must exert himself even to a small extent, such as go to the market and purchase the item, the prohibition against facilitating the violation might apply.
Although this entails a significant limitation on the definition of “two sides of the river,” a similar position appears in Shut Chavot Yair (no. 185). Following similar lines, the Pischei Teshuva (151:2) cites the Emunas Shmuel that the concept of “one side of the river” only applies when the sinner can obtain the object on his own, and not when he must purchase it from others. However, the Rema (151:1) writes that the principle applies even when he must purchase the object.
