Are there any websites where I can listen/download a Gemara shiur in English with rishonim, acharonim at a relatively high level?
I'm not looking for daf yomi bekiyus shiurim but rather a deeper more iyundik one and preferably one with marei mekomos to help prepare for the shiur.
I quite like the gemara shiurim at Rav Nissan Kaplan's Daf Yomi Review. Many of them also include mar'ei mekomos as a .gif file - such as these ones, from the first and fourth perakim of Gittin. I would consider them to be reasonably high level (for me, certainly) - at least insofar as they look be'iyyun at a variety of different Rishonim.
There are probably thousands of shiurim on Gemara, as many 'maggidei shiur' will upload their lectures on the internet. There are two general ways to find them:
by looking for a particular yeshiva or teacher, such as the audio library of Yeshivas Ohr Reuvain, Rav Yitzchok Lichtenstein (both of those are rabbis in Monsey) or Yeshiva Toras Moshe, for example.
if you don't know (or care) about the particular lecturer or yeshiva, there are two free online databases of shiurim, both of which are huge and excellent: http://kolhalashon.com (which has thousands of excellent shiurim from many lecturers, including world-renown roshei yeshiva) and http://yutorah.com (with shiurim from lecturers who are associated with Yeshiva University, and many which come with PDFs of 'mareh mekomos')
