Aside from the safety issues about not using a transformer...
if you can't get ahold of large power resistors (which are relatively easy to get compared to other components), how do you expect to get the parts for other methods? Also, a couple of 5 or 10W resistors also easily take up the space of 1x2 inches.
I would say a capacitive divider possibly followed by a diode rectifier. THe capacitors might be small enough. But I doubt it. THe problem is that you don't seem to have space/experience to use switching or transformer methods. So the only thing left is linear methods. (120V-15V) * 0.1A = 12W that you have to burn off. So the resistor size (or capacitor size) all sound about right. Can't get around the laws of physics.
But the only way may be a switching converter. THe easiest thing is to buy a premade one (and these ones also have isolation transformers so they are safer).
Like these from Digikey:
**broken link removed**
**broken link removed**
But remember, no transformer means no isolation means lethal shocking potential. ANd using dividers of any kind also makes for poor output voltages that can swing wildly on load changes.