Available from Fowlers.
Yamaha DT230 Lanza Airbox duct. Really nice upgrade, much larger duct for more air intake. Jet slightly richer towards 3/4 Full throttle and this Duct will work very well.
A little material has to come away to seat it's larger flange, so be sure, and commit to jetting if needed.
Better sound there too.
Airbox Tract. 1988-98'
The later Airboot YIES bottle was introduced for better low RPM induction and emissions. It works really well for the Midrange, but makes the bike a little flat on top.
For Top end, I use the older Airboot without YIES. It works better for higher RPM.