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.
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 98. The previous thread is at nearly 500 comments.
There is now a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system.
Also: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: