The Cubs and Cardinals, three years later than originally planned, will play at London Stadium in England June 24 and 25, 2023. (They had been scheduled to do this in 2020, but that was cancelled due to the pandemic.)
Today, MLB Europe posted this informative set of pages about the series.
Cubs and Cardinals season ticket holders will have a presale for tickets to these games beginning November 29. The next day, another presale will be held and you can register for that here. The site says that tickets for adults will be available for as low as £54, which is about $64 at current exchange rates.