Food for thought from Rob Watts himself:
“Actually my advice is that the supplied PSU is as good as a battery supply (based on my listening with my set-up -YMMV) - which gives the lowest amount of RF noise - and that LPS supplies is likely to sound worse, as they are often transparent to RF noise from the mains - the supplied switcher has RF filters. My advice is to try a USB battery bank, and if it sounds identical to the supplied PSU, then don't bother "upgrading" with a LPS...
If you still wish to pursue, try this LPS which comes with a 30 days money back guarantee.