My favorite Yamaha dealer: http://www.sanjoseyamaha.com
Click on showroom,
Click on motorcycles
Click on view yamaha parts
Pick a year between 2008 and 2011 and click, say 2009 (Other years are more expensive.)
Scroll down to TW200Y/TW200YC and click on it
Scroll down to seat and click
Seat assembly part number 5VC-24770-10-00 should appear. Cost $100.86 Brand new. Will it fit a 1999 TW? I don't know but I bet it will. You might ask about the color too.
I think Bike Bandit has the same prices. And check your local Yamaha dealers. Maybe the offer the same deal, especially if you show them what SJ Yamaha is doing.