The AC adaptor to my Asus EEE PC, 1000HE (purchased in the US) has been lost and is unrecoverable. Can anyone please tell me where in Thailand I can purchase a replacement charger? Preferably an original asus, but anything that would do the trick would be okay in the meantime. I can find some online, but it is already expensive enough so it would be nice to avoid some significant shipping charges.

Any shops in Pantip, IT square, Fortune?

J Leang in IT Square sells a lot of different adapters. I'd imagine that the other places you mentioned should have them too.

The sticker on mine describes the output as 12 V, 3 A. You should be able to find something to work easy enough.

Yeah just dig through the boxes of random replacements till you find one, same voltage (within half a volt or so) and at least the same amperage (can be higher, doesn't matter). You may even find adjustable universal ones.