I've never heard of one. Computer power supply is regulated. To reproduce it in linear fashion it would have to operate at minimum line voltage and be able to take max line voltage. Since there is perhaps 20% difference between max and min linear regulators would have to dissipate about 20% of rated power. For 300W supply it would be 60W. Taking other losses into account it would be close to 100W. Removing that much heat, even with a fan in small cage seems impossible especially if you take into account space needed for capacitors and large transformer since transformers operating at high switcher frequency are 10 times smaller.
I assume that your intention is to reduce jitter on digital out. The best solution would be to reclock before DAC or use transfer method that removes timing (Ethernet, Wireless etc) and then reclock only if necessary. Good computer audio board most likely will have own supply that is isolated (floating) but I don't have much experience with audio cards.