I can recommend a good supplemental book. It's called "Speak Business English Like An American: Learn the Idioms & Expressions You Need to Succeed on the Job!" by Amy Gillett. It comes with an audio CD.
I've seen other business English books, but this book is really practical and made for people who need to get up to speed fast. Of course, your English should already be at a fairly high level before you start studying business English.
As for how to learn business English on ITALKI, I've read profiles of English teachers who specialize in business. Look under the tab "Language Teachers" for them.