I found a good page with a pattern and how-to for a manica.
http://www.larp.com/legioxx/manica.html
Here's one in leather:
Are you interested in the whole Roman getup? Your breastplate looks quite a bit like lorica segmentata, and there are numerous websites with patterns and how-tos for it. I might even be able to hook you up with some hardware.
The Legio XX site is a great resource for roman stuff.
lorica segmentata: http://www.larp.com/legioxx/lorica.html