Most of the ideas listed so far are either much too nasty, better done with magic, or both. Mostly both. So, what contingencies could Harry procure from the world for a reasonably modest amount of money that do things he cannot just transfigure, or do by wand, and which the twins would not recognize?

First thing that comes to mind are various non-lethal drugs, since those are going into the targets body, which means transfiguration is out if you want the subject to live. So, LSD (lethal dose absurdly higher than effective dose. .which is useful if you want to dose a crowd) Tranquilizers think this fits on all counts?

Lets see, what else would it be a bad idea/impractical to transfigure. Muggle photography gear? But the twins would recognize that.

Smoke grenades. (Not subject to finite, a counter for every spell that requires you to see your target.)

Protective kit good enough to count as valid precautions for transfiguration stunts.. would break the budget given.

Id say bugging kit, but a purely mechanical recording device would be a museum piece, and cabinet size..