Some time ago I lent my wonderful HP Core2Duo TC-4400 “Hybrid Tablet” to a friend.
“Just a few hours, really, I need it, don’t worry”
So I lent it, it was just an afternoon.
The night I got it back, I opened it and I started to type, everything looked ok… until I had to type “?”… the key falls in my hand!
“What the…?????” I shout, on the phone
“Oh, a key fall off? Really?”, they say.
I get to know that an ormament fell on the key from a shelf, breaking the retaining hooks on the key and on the lever.
“That’s so bad”, I was thinking… “this is a model I bought in the USA, it wasn’t on sale in Italy, I wonder where I can find the right replacement, and how much it will be!”
I used the keyboard without that key for a week, then I Googled for a fix.
I found laptopkey.com and replacementlaptopkeys.com that will send a replacement for less than 5 euro!
Finding the right key was hard, because my model is rare, and it wasn’t listed on the website. Moreover, every model has 3-4 different kinds of retaining hooks, so you have to carefully look at the pictures on those websites…
After a week I got an envelope with my key. Another little trouble: in order to protect it during shipping, they preassembled it, so you have to carefully separate the key and the lever to mount it. 
Once mounted, it’s great!