Technically that should be 'Where are the...', 'what are' is a whole different thong, unless you're inquiring into the nature of these agencies. You might want to watch that.
Are you looking for a teaching job, or are you looking for work in one of these agencies? If the former then I assume you've tried the Jobs section here?
I g**gled, 'teacher recruitment agencies in Hanoi' (lateral thinking) and found stuff, most of the info leads back to this site. I'll give you Teach-It! Recruitment for free, but there's more info out there. Give g**gle a go.