Looking for illegal tour operators in Beijing!?

I went to Beijing several months back and paid 70 yuan for a half day tour to the Great Wall and Underground Palace. It is somewhere near Fu Xing Men train station but am not exactly sure if I remember. We had free lunch as well and I think it's a good bargain. It's quite a big operation and many mainland Chinese take these tours too. Can someone tell me where exactly is this place and how can I get there or if there are some other better illegal tour operations with a better bargain?