You're thinking like a normal person. You need to think like an institution that has the entire weight of government behind them and who nobody wants to be on the wrong side of.
They either find someone who has suitable vans they can threaten with prosecution. That person then agrees to be an "informant" because that's better than losing your life to the feds and then their handler asks to borrow a van. They like this because no money needs to get spent specifically on it so it doesn't tend to get scrutinized. If they're going full above the table they register a business with the state complete with valid HVAC license or whatever and then rent a van from some company the FBI owns/runs that rents white vans and have some decals printed up. (For those inclined to do further reading, the OSINT hobbyists have done a lot to expose this workflow as it relates to aircraft so probably start there.)
They either find someone who has suitable vans they can threaten with prosecution. That person then agrees to be an "informant" because that's better than losing your life to the feds and then their handler asks to borrow a van. They like this because no money needs to get spent specifically on it so it doesn't tend to get scrutinized. If they're going full above the table they register a business with the state complete with valid HVAC license or whatever and then rent a van from some company the FBI owns/runs that rents white vans and have some decals printed up. (For those inclined to do further reading, the OSINT hobbyists have done a lot to expose this workflow as it relates to aircraft so probably start there.)