I thought this might be useful as a topic in its own right.
Blythe and Pullip hats fit Tangkou well.
As for clothes, modern Barbie/My Scene clothes usually fit great, as do most Liv doll clothes and Momoko clothes.
Blythe clothes are generally too small and Pullip has smaller boobs so tops can be too tight, though some stuff fits.
As for shoes many (though not all) Liv doll shoes fit Tangkou as do some Sindy shoes, I also found out the other day that some shoes from the smaller fashion doll sized Best Friends Ink (BFC INK) dolls will fit too.
Hope this helps, if anyone has any other suggestions then please post, we'd love to know :)