If you can't find it (have you checked the downloads section here?) you can use DSTT firmware, just make a copy of the ttmenu.dat file and rename it to u2ds.dat
so on the root of the mem card you would have...
ttmenu.dat (still needed for soft reset to work)
ttmenu (folder and contents)
u2ds.dat (renamed copy of the ttmenu.dat file, needed for the u2 cart to boot)
using the same method you can also use the y's menu which is what the real u2 firmware actually is anyway.