The Precision Scale catalogs are now online. Click on the "Detail Parts" tab and you should see a list of them there.
I have used P&D to source PSC parts, as recently as this "March" meet in Chicago. I ordered 10 or 15 different PSC diesel parts and Pat was only missing one. And he was kind enough to deliver them to me at the show so that I also saved postage. And he could have ordered the missing part if we hadn't been on a schedule.
You can check Caboose Hobbies as well, but my personal experience there is that their stock is less comprehensive than P&D's. I'm sure they can order, but I don't feel like they have anywhere near the inventory on hand.
P&D also carry Cal-Scale parts, some of which are no longer listed on the Cal-Scale/Bowser site. And if you need 1st generation diesel parts, they have a fairly extensive house brand of parts for F's and early gp and rs units.