As another commenter mentioned, diesel fuel and fertilizer are ubiquitous. Tannerite, as you specifically mentioned, can be purchased in bulk by tens-of-lbs at single stores. In fact, you're likely to save a good chunk of change doing so.

I'm not being sarcastic when I say that the US is awash in explosive materials.

If you’re a farmer, plenty, as the components for one compound are diesel fuel and fertilizer, both of which are part of “running a farm”

I think people way overestimate the FBI and police department capabilities for detecting these activities.