Hello, a question to the numerous Bike experts on the forum --

Some time ago, I bought a second-hand bike with 3x6 Shimano gears. Recently, the "front" gear shifter (the one with 3 front gears) broke.
This device is a circular plastic sleeve sitting on the handle bar, which I turn in order to pull the gear-shifting cable. Now the question - can one recommend where can I buy a spare and how easy or difficult is it to replace this thing? Being a second-hand inexpensive bike, I am not too keen on spending a fortune by taking the bike to a bike shop for repair, which might in all probability cost more than the cost of the bike itself. And, more importantly, I am quite a novice and know little about the art of bike repair.
Thanks and regards!