How do i pick out a transformer?

I'm finally ready to build a power supply which doesn't appear to be
to difficult. But, I am having issues picking out a toroid
transformer. I know i have approx 110VAC or the wall socket, and I
need 100VDC and 20amps, which is the max for a rutex drive of course
slightly underpowering my motors which i feel fine about. I'd rather
under juice them than have the ability to over juice them. So when I
look at transformers what should I look for, to get the desired
output? I can handle the rest with the bridge rectifier and
capacitor.