I use ChieP's romantokana plugin, but I just searched for the place I downloaded it from and I couldn't find it anywhere. The readme says that redistribution is allowed, though, so I'll just reupload it. Here you go:
http://www.mediafire.com/file/k0sk6yw0rk4t0wq/romantokana.zip
Just make sure that when you run the plugin, you're using a CV UTAU that has both hiragana and romaji aliases, and you should be good to go!