"Lewis Drum Magazine $385.00 Add to Cart
47 round drum magazine for .303 Lewis Gun. Original. "
http://www.collectorssource.com/category.asp?catid=9
Collectors source has one listed right now. You might find one cheaper out of the country, but this way you don't have to mess around with the import permits/customs/etc. Otherwise IMA has them listed as well, at $210 US each, or 3 for $499.95 US.
http://www.ima-usa.com/index.php/cPath/1_214_110