No offence, but comparing asking for proof of corruption with proof of sky being blue of petrochemicals is a biased bad faith argument.
Asking for sources on corruption is more like asking for proof that the earth is round, which is definitely not a legitimate request, but more trolling masquerading like an innocent request and dodge scrutiny ("It's just a question bro").
Nothing wrong with asking that question per-se, but that's something you can also google yourself due to countless occurrences from legitimate sources, hence why it's in bad faith to ask such a thing, and should be more strictly moderated as many here abuse this "sauce or gtfo" attitude in bad faith.
Existence of corruption isn't what you asserted.
>> how much taxpayer money governments loose via waste and corruption
That's the assertion you made -- waste and corruption at scale.
It's very much a reasonable question to ask for sources of how much there actually is.
Otherwise, people just post things on the internet insinuating that there's a huge (unspecified) amount.
Is it 1% of the budget? 5%? 25%? (Hint: it should be trivial for you, the claimant, to dig up a source. And it's close to one of those)
If I say the sky is blue because of plane chemtrails, and you ask me for a source, that seems valid.
As with any large procurement system, there is moderate government waste in proportional terms, but one of the primary drivers of that waste is... anti-corruption systems operating as intended.
If you require 4 more forms than private sector, in order to be more sure there isn't corruption, then you've just imposed a cost that creates no value.