A week ago my rear mech fell apart. Shimano, aren't they terrific🙄 . I know very little about bikes. I have a 21 speed bike, one of the 7 gears is a megarange (though I never use it) I have EZ fire shifters and three "big gears" on the front. I need a replacement rear mech but don't have a clue what I'm supposed to be looking for. I am in the UK and I am a student so it needs to be pretty cheap if poss. I've been looking for the "Shimano C030 MegaRange" which is apparently what my rear mech was called, however I can't find one anywhere.
You're looking for a derailleur. It looks like something that primarily went to the UK, but a bike shop should be able to find something here that may work.