With the following assumptions:
1) your scope is NOT ground referenced. ie, it is fully floating, and you have nothing connected that could be a "bridge" to ground (ie a charging or data lead etc)
2) You ensure your own personal safety by the following precautions:
a) some decent high voltage gloves
b) a non conductive work bench (kitchen work top or wood, no metal!)
c) an appreciation that touching something might kill you
3) you only need to measure a relatively low frequency AC component (you say transformer, so i assume 50 or 60 hz fundamental)
Then you can make a simple, cheap, passive restive divider out of some suitable resistors (daisy chain lots of resistors together to get the overall impedance and ideally pot or seal those resistor chains to avoid arcing or touching. a 100:1 divider is what you're after. The impedance value will depend upon the input impedance of your scope (see the datasheet for it) but use an external low side resistor, don't rely on just the scopes impedance (but make sure to include it in your resistive divider calcs. You can "calibrate" the divider with a simple, safe low voltage dc source, like a 9v battery before you use it on HV stuff