Which episode is this where Saber dress in white and bounded? In Unlimited Blade Works, which episode was Saber bounded up and was in a white dress being raped by Caster?
Saber wears - or more exactly is made to wear - those clothes in Fate/Stay Night: Unlimited Blade Works episode 13, 14, 17 and 18. (Or season 2 episode 1, 2, 5 and 6, if you count the series as two seasons.) 

The screenshot itself is from episode 13 (or S2 episode 1).

By the way she isn't being raped (not even in the visual novel). Caster just tortures her to make her lose her mind so she would want the pleasure by herself.