I'd assumed it would be a variety of muggle items that were generally useful but hard to get, rather than preparations for any one specific thing. My "mental image" was basically a chemistry set, since Harry probably knows some tricks that require specific chemicals that you can't get at a normal shop because most people don't want them.

That wouldn't require going outside Britain, though. Something illegal or highly controlled, like Izeinwinter's suggestion of LSD, or the weapons a lot of other people are suggesting. I don't /think/ Harry would go for warheads, but guns maybe.