There's a lot of laptop hardware out there that is hard to find. Since my store doesn't stock a large variety(just the common hardware) we have to search the net to find replacements for a lot of proprietary parts.
http://www.laptopparts.com has the common stuff (power adapters, batteries, etc...) for a decent price and we've ordered from them before.
Another site has just about everything (I'm sure you could build a complete laptop from their inventory), but we've never ordered from them so I don't know about reliability. http://www.sparepartswarehouse.com/