This site can be quite good, it has easier and harder lessons and audio, this one for example has a lot of idioms. http://www.breakingnewsenglish.com/0505/050514-rockrat-e.html

You can look at scholarly articles and academic sites according to your topics of interest. This will increase your vocabulary and you can learn idioms as well. Choose more advanced level teaching resources or those relating to business.

I suggest you to purchase a workbook appropriate for your needs. I know of this book called "CAN YOU BELIEVE IT" and there are 3 volumes depending on your level. You can learn new idioms and how to use them in sentences. I suggest you to also ask a tutor or teacher to discuss it with you so you can express yourself through the different activities from that book. I have taught this book to Koreans of different ages and I could say it was effective. I hope it will be for you too!