Not sure about the power required by the D50, but if it's <1A, then you can find on diyaudio forum at least 2 guys manufacturing low-noise PSUs. Checkout is done by eBay and I can give you at least 3 people selling very-good quality PSUs if you like, not mentioning DIYTHINK...this is the 4th one.
Then follows a nice Hammond case, a toroid and few connectors + a mains filter inline with the power cord + 2 x small ferrite across L/N. Definitely less than 100USD, so no need to spend more on the PSU itself.
However, even a LT1085 or NJM7805 could do the job very well. I got after the NJM7805 less than 100 uV AC ripple& noise, which is very good actually, because the DAC itself and around opamps are already having ripple & noise reduction based on internal PSRR/CMRR specs.
Also, an isolation transformer might help too in decreasing the noise coming from the mains, especially if ground is filtered or perhaps lifted correctly (eg.: Hum-X).
P.S.: I always measure all USB port's noise before connecting my DAC, then I choose the least noisy one. I usually get less than 1mV p-p noise (200-300 hundred of uV RMS, depends) which is quite OK for me...well, I don't get any noise injected in my headphones.