I don't know of any source for an exact replacement for any Kustom power transformers. What is the problem with your transformer?
You mention 40 volts for the secondary, but I think that you are referring to the power supply dc voltage of plus and minus 40 volts dc. The transformer output will be less than that, probably about 30-35 volts ac per side of the center tapped secondary.
You may need to find a used one from a dead amp to replace it. There are companies that will custom wind or rewind power transformers. You might look into that.