Ask the Mauritians
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Where can I buy key retainers for my laptop?
Otherwise you can go into a local computer repair shop to see if they have a spare one for you.
If none of the above work for you, you can try ebay but you might be more successful in buying the whole keypad rather than an individual key. To replace the keyboard on the laptop is not difficult; there are some really detailed how-to videos on YouTube to show you how to do it step by step.
If the glue did not work, why don't you swap a key that you don't use very often with the broken one? This link will help you in the swapping process :
Or like I said before, try going into your nearest computer shop (repairs) to see if you can get a used key retainer.