How to get the Public Keys of a bitcoin User Is there some way in which I could get the public key of a Bitcoin user ?

Lets say I have transaction with another Bitcoin user. Is there some means by which I can retrieve the public key of the Bitcoin address they used in our transaction?

How does https://slock.it/ guys does it ? From where do they get the public key from ?

https://www.youtube.com/watch?v=-Ht23KXic1k
You cannot compute the public key of an address (that is not yours), because the address is computed from the hash of the public key. 

You can find the public key of your own address with the Bitcoin-QT debug window or with a validateaddress RPC call.