Lets say I have this code.
Not verified, just a hint:
var v = document.getElementById('<%=CustomValidator1.FindControl(CustomValidator1.ControlToValidate).ClientID>%');
of course you could simply do it like:
var v = document.getElementById('<%=TextBox1.ClientID%>');
if you know exactly what you're validating. The first method is good when the control to be validated is set dynamically and you don't know beforehand which one it will be.
Also FindControl() might return null so you'd need to test for that too in order to avoid an exception.
Hope this helps.