Anyone have the part numbers that parts houses (NAPA, Advance, etc) use for the 5204 2rs and the 6204 2rs bearings?
Yes, and so do you.

Those numbers will get you what you want at any decent bearing supply.
Does not matter who made them they all use the same sizing.
Definitely want the 2RS and not ZZ
RS is seals while ZZ are shields and they don't fully seal
But the suffix shows the rest of the story, as Blackduck pointed out. You want dual seals.
Have you researched the dual bearing mod?
http://www.valkyrieriders.com/shoptalk/5204_bearing_mod.pdf