The new Siglent 4ch low cost SDS1x04X-E scope series might do it too if you have/buy a suitable sig-gen (in a similar way to Keysight, scope has bode plot function built in). If this is a one-off then maybe you could do it manually, measuring with scope FFT + sig-gen at a range of frequencies. Bandwidth requirement won't be that large, a few MHz at the very most. The thread about Dave's Bode 100 teardown video also has info about a bunch of DIY and low-cost isolation transformers (short answer - wind one yourself or find a video isolation transformer).