There is a known problem with the built-in search engine for the forum, and sadly it's not anything that will be easy to fix.
Your best bet is to use Google with the modifier "site: photonlexicon.com". That usually gets you to where you want to be.
Here are two sites that weren't listed in the thread Kecked linked you to that I still like to check whenever shopping for PSUs, even though dual-polarity supplies aren't nearly as common on these sites:
https://www.allelectronics.com/
http://www.goldmine-elec-products.com/
And of course, it never hurts to look on E-bay and Amazon as well.
Adam