You are just falling into the US's trap where they create these narratives so their opponents self-limit themselves.
Russia never denied it bought Israeli drones, or any of the other things it bought, why stop believing them now?
The west has openly poured money and weapons and ammo on Kiev, which as you point out, much of which they had to outsource because they couldn't produce it themselves... yet the only people suggesting Russia is buying from other sources are those same western sources who had to do that much earlier in the conflict.
Sounds to me like Russia and Iran are telling the truth and the west is lying to cover up the fact that they don't make weapons any more in the volumes needed for a real conflict.
Their boutique wars to steal resources or to murder rivals consume rather less ordinance it seems.
There is none. And who cares. I think Russia should be using the Fateh series of ballistic missiles actually. It is likely way cheaper than anything Russia has right now. And they should get Belarus to sell them the Polonez. They are already sanctioned anyway.
I keep hearing people say Iranian weapons are much cheaper than Russian weapons but I never ever read any sort of break down to prove it.
Modern ballistic missile do require chemicals and electronics and equipment to make them and not all are cheap or even readily available, and if Iran does not have its own supply then it would probably need to import such things, which no doubt Israel and the US and the west in general would try to restrict.
The cost for making such things in Iran wont be as high as the west because Iranian arms manufacturers are not criminals like they are in the west, but considering their situation, the costs are probably justified by the threat from Israel and the US and the general hostile west, but I rather doubt they could sell such weapons at give away prices... especially to a country as rich as Russia in comparative terms.
In fact I would think deals would be made where Russia might be trading GLONASS sensor chips and modern rocket fuel and other bits and pieces... even small electric servo motors and electronic components in return for licence production of Iranian drone designs that are proven to work and they have likely tested over the last few years in Syria.
Most military are the same and when they find themselves in a conflict zone with allies they will always test each others gear within reason and interest in the kit and equipment the other guys operate will always be high, whether it is superior or inferior, it is interesting to have a look and to play with something exotic.
There will always be individual features you like or dislike whether the kit is superior or inferior to your own that could be passed on to your own makers to improve your own equipment.
Sometimes if their kit has many desirable features it might make sense to just make a local version of it and licence produce the type with local parts... so you can maintain it and keep it going.
I don't think Russia would buy Iranian stuff because it is cheap, I would think they would make it themselves so it probably wont be as cheap as the Iranian products till they have cranked out hundreds of thousands of them because they have to include the cost of the factory to make them in the cost.
Which is not to say Iranian and Chinese and North Korean and even Cuban or Venezuelan or Vietnamese gear should be ignored as inferior because their designers and engineers are not idiots... for all the high tech up to date capacity of western anti tank and anti aircraft weapons, they don't seem to be that amazing... I would say Kievs existing ATGMs from the cold war and post cold war period were probably more effective... without even taking into account the price.
Edit: now this just occurred to me. Soon people will claim Iran sending their fighter jets to Russia as Russia needs jets and can't build them.
The irony is that most would agree that that is silly, however I would not under estimate Iran, they have invested a lot of time and money and experience into drones and ballistic weapons and seem to have managed some significant progress.
The problem with their ballistic weapons is that they are ballistic, they don't manouver like Iskander does, so Kievs Buk and S-300 wuld have a good chance to shoot them down, unlike with Iskander which manouvers to avoid interception as it comes in to hit the target.
Their drones are also rather interesting and capable, so I would say the drones are probably best worth a look... the gap in the ballistic weapons category only exists because of the INF treaty on ground launched medium and intermediate range missiles (ballistic and cruise).
It should not be a huge problem to fill that gap for themselves, ground launched cruise missiles would likely fill the gap first...