One of the best ways of getting access to Amiga kickstart ROMs and
games completely legally is to buy the Cloanto Amiga Forever CD (see
http://www.amigaforever.com/). To use this with UAE-wii, you'll need
to do the following (works with version 3 and above):

1. Copy everything from the "Amiga Files/System/rom/" directory on
   your Amiga Forever CD to /uae/roms/ on your SD card.

2. Copy /uae/roms/amiga-os-130.rom to /uae/roms/kick.rom

3. If you have upgraded from an older version than 3, remove
   /uae/uaerc.saved from your SD card

After this, you can start UAE-Wii as you would otherwise and it will
use the Amiga forever kickstart ROMs. You can copy floppies (ADFs)
from the CD in the same way to /uae/floppies/.