It depends on the quality desired and how much you want to invest.
Perhaps the most bang for the buck would be the Olympus B-300 1.7x - hard to find - but try:
http://www.eagleeyeuk.com
If they don't have it now, they will soon.
Next, would be the TCON-14B 1.45X - large - fairly expensive - heavy - excellent quality. Widely available on the web.
Next, the TCON-14 1.45X. Not quite as sharp as the TCON-14B, but still a good value. Much lighter and smaller and about half the cost. Available at most Olympus dealers and on the web.
Lin