Almost as good as the K-H 4402 is the -hp- 339A THD analyzer, which includes a low-distortion generator. The residual distortion in mine measures about 0.002% (-94 dB) which should be good enough for any normal applicatioin. Units such as the 339A have the advantage that the analyzer tunes directly to the internal oscillator frequency, and will lock to an external oscillator when the frequency is approximately correct. Also, the indicated fraction of distortion scales automatically to the input signal. You can learn a lot about the DUT by analyzing the distortion output signal, which has the fundamental removed.
The oscillator part of the 339A was made as a standalone oscillator model 239A - quite a small dinky little thing, and no fan so they tend to be in immaculate condition internally. I have one and it works nicely, but they are
very rare on the secondhand market. As well as low distortion, it has good level setability right down to sub 1mV, and it the output is isolated from ground - two essential attributes for a low distortion audio source.