There are heaps of sellers on GlobalSources.com direct from China.
Do you want a player with a screen that plays MP4 videos as well or just a very basic MP3 audio player only? There are lots of options.
Actions are the most common and generally more cheaper than the rest of the solutions used. Rockchip is nice, but alittle more costly, same as the Telechip (Teclast) based players.
Did you have any in mind?
I think you also mean 256MB and 512MB and not KB right? I don't think you can get a 256KB/512KB chip anyway