Parts like that from HP are usually unavailable as you discovered and you may have to get an entire printer (dead or alive) to scavenge these. Once in a while you encounter folk that only want the parts they want. If so you, as the shop keeper get it from ebay, sell it to them for the usual markup of what the entire printer cost then ebay what remains.
Bob
Hello,
I own an HP Deskjet D4200 series printer. Recently, my input and output trays were broken in a freak accident, and have to be replaced. I looked at various sites, and couldn't find the parts.
Are they replaceable, and if so, how much does it cost?
Thank you.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic