While Tourist Israel is not government, it's a pretty reliable site and their page has this important note:
As of January 2019, Israel made a change to remove cash from bus transportation and use the Rav-Kav card exclusively.
OK so we need that card! Almost everyone will enter Israel either at Ben Gurion or Ramon so let's see where we can get a card after landing...
The same Tourist Israel page mentions "For travelers looking to purchase anonymous Rav-Kavs, there are several designated locations around Tel Aviv including all of the Tel Aviv train stations, the Central Bus Station (level 6) and in the arrivals hall of Ben Gurion Airport. To purchase a Rav-Kav is 5 nis."
Ben Gurion Airport arrivals hall, that ain't a lot of help. This tripadvisor thread to the rescue:
Starting Monday 16/4/2018, Israel's Ministry of Transportation has opened a Public Transportation Information Center which also issues Rav Kavs, Israel's public transportation electronic card. The information center is intended mainly for tourists.
Opening Hours: Sun-Thu 07:00-23:00, Fri 07:00-15:00
Location: Ben Gurion int'l airport, Terminal 3
and links to the government page in Hebrew which has this photo:
And we have a user reporting in the comments the machine next to it sells cards and that's likely open 24 hours.
Now, Egged itself obviously has a Rav Kav Card page which lists all their locations, including one for Ramon airport (notably Ben Gurion seems to be missing from their list but we covered that already):
Ramon Airport - in front of arrivals exit
Sunday to Thursday 7:30 - 22:30
Friday and holiday eves 8:00 - 16:00
Saturday/holyday - from the "going out" time and till 22
Personal note: thanks for the question, I would've never thought they stopped accepting cash and would've looked silly when I try to board the bus in Eilat in January, now I know and I will buy a card at Ramon when I land.