Have you contacted either vendor (maybe through your FAE) to see if they can supply a model? I looked at the ON website and there are some IBIS models there, but not for the parts that you're looking for. I only saw PSPICE models.
For National, I found this site, maybe one of the LMK devices is a close enough approximation to the output stage of the device as the LMX2541. You could ask them about it...