LOL, there are records proving he had traveled to Pakistan several times and phone records to the man he was visiting in Pakistan. Cant remember his name but he was an expert bomb builder. Same dude that build the first bomb set off in the parking garage under the WTC. I would say he had vetted support based on that alone.I don't remember McVeigh having some vetted network to rely on.
At the time he pulled his bombing, you could obtain specific instructions off the .gov websites to build mini bombs using Ammonium Nitrate and fuel to blow stumps from a field being cleared.
He just went one step bigger.
This guy could have very easily researched the internet to see what to do. Its no secret needing some big organization. It's not even illegal to buy or possess Ammonium Nitrate currently. Most fertilizer supply COOPS quit carrying it though because of the massive amount of paperwork it takes to sell and and the paperwork involved in buying it.
It's actually a better fertilizer for the farmers.