There have been a quite a number of threads on here discussing 21.1mm stem and possible solutions like machining one down to the correct size.

Alas, a simple solutions has emerged. I wondered why this hasn't come into production earlier.

I came upon this on ebay.
http://cgi.ebay.com/Bicycle-Bike-Ste...item335b8c579f

Unless, you are intent on using a quill stem, this is perhaps the easiest and most practical solution for a 21.1mm. YAY!!!