American equivalent of "Flabbergasted"

I cannot really use it when it's an American talking, as the word is primarily used by British people.

Are you sure about this? I just ran a quick search and this is what I found:

• 440 hits —Corpus of Contemporary American English (corpus contains 560 million words total)
• 44 hits —British National Corpus (corpus contains 100 million words total)

The evidence indicates that American English uses the word more often than British English. Furthermore, if you look it up in pretty much any dictionary, it is not marked "chiefly British".

As an American, I can tell you that I would use "flabbergasted" and I do know what it means.

So, my answer is that the word you are indeed looking for is flabbergasted.

I do believe "dumbfounded" would be more suitable to that sentence than flabbergasted.