Well, you can do this:
1) Hold down "Alt" key
2) Type 0, 1, 8, 9 (while still holding Alt key)
3) Release "Alt" key... tada~
Your IME must be set to ENGLISH! Not Chinese, not Japanese, not Korean but ENGLISH!! lol
Here goes: ½... and ...¼ (0188), ¾ (0191)