there's plenty of boxes about for under £100 but you won't find a carrier box with the correct gear selector housing, (instead of a gear stick it has a stubby splined shaft that the linkage attatches too), this is easily rectified on a normal selector by cutting the stick down and using a grub screw to secure it in the right position. you will also need the dished flange that goes on the back of the box and butts upto the axle ( about £100 to have it made )also the straight spur gear that goes between the g/box output shaft and the diff input pinion, you can get these nos from dirk leegwater for £30.
however having said all that i've just remembered a guy in the states that might have one for sale if you don't mind paying the shipping. send me a pm if you want his details.
dirk's site http://www.lwdparts.com
also try this bren carrier site www.mapleleafup.net
rick
ps i'm in the process of restoring my own t16 if you need anything give me a shout