To be clear, this is not (just) for a power supply. So some flexibility would be nice.
Yes, there are FET probes on ebay, but like you said, there is a risk that it's damaged or doesn't work to spec, and there is no good way to know.
The FFT on a scope is pure garbage compared to a SA. It's difficult to use and not very useful.
There seems to be a market opportunity here, and I'm stunned it hasn't been filled. Really, I'd spend $200-$300 for a proper solution:
1. Connect a regular oscilloscope probe to my spectrum analyzer. Or some simple similar probe
2. Include DC block and output overvoltage protection (a cap and diode?)
3. 100-500MHz bandwidth (maybe?)
4. Possibly include some scaling (this isn't even needed)
How is this not a thing that exists? My ancient, beastly HP SA even has a separate 1Meg input for a probe. (But it only goes to like 40Mhz, is very slow, etc.)