In your first post, you said "We want to use our scope for testing line voltage." Why can't you just use a voltmeter, a handheld DVM for example?
Perhaps you need to observe the waveform of the line voltage, but you didn't say if that is the case. Assuming you really need to use a scope, and if you're looking to buy a new scope, you could get a scope with isolated inputs and which is intended for measurements on grid power service. Such a scope is the Tektronix TPS 20xx series:
**broken link removed**
You don't need to use differential probes with this scope; the 4 inputs are all isolated from each other, and the scope can be battery operated for greater safety. You need to use the right kind of passive probes for safety, and for voltages over 120 VAC you will probably need to use 100:1 probes.
With a 4 channel unit, you can observe all 3 phases at once with no further isolation other than what the scope itself provides.
Alternatively, rather than using an isolation transformer to power the scope you could use a transformer to isolate the voltage you want to measure. To get good accuracy you should use a potential transformer which is specified for the purpose with known accuracy, for example:
**broken link removed**