Do you mean the .833" size? If so the answer to your questions is: you don't. The only thing you can do is try to fine one on ebay or buy a crappy steel wald stem in that size.

I took a stem off of a super sports and it's sort of i-beam looking- aluminum with a flat top, the i-beam being the underside of it, and it solved my problem. If that's the sort of stem you're talking about that's the best you can do most likely. I've found .833 stems on ebay but long ones are very rare.