Not that easy, their entire military is based on western arms, retooling and retraining your entire army isn't that easy, the retooling part is especially not cheap GL hacking your Russian guided bombs (if they agree to sell them to you) to work with US aircraft, sensors, and command and control tech etc, yes it's possible but it's not like well gosh we can't buy US cars any more, screw GM, MG here we come, you can't just switch over.

Also both China and Russia don't just export anything, in some cases it would be harder to buy from them than from many Western nations.

vendor lock-in ftw

Yep, there is no lock-in like in the defense industry, mostly because you want to make sure who they can later "lock-on" too ;).

Oddly enough of the US some how stops selling guided munitions to Saudi Arabia, Israel is probably going to replace them. Their weapons work with both NATO/Western and Russian systems, and Israel is reportedly already supplying weapons to Saudi Arabia mainly white-labeled drones through South Africa.

So, when it comes to money, it's totally irrelevant how much of a disagreement the Israeli and Saudi governments are.

I wanted to suggest they could buy from their neighbor Israel, though didn't think it's possible, but honestly they could also buy from India or Iran.

Iran and Saudi Arabia are "mortal enemies" on many levels, India is an option but it co-develops allot of its weapons with Israel.

Also don't confuse overt and covert politics in the Middle East. There's allot of posturing and pretense on the surface but allot of cooperation under it. Israel and Saudi Arabia are as close to allies as they can be in the current geopolitical landscape.