Bike broke, strange parts came out, and doesn't know where to find exact replacement My friend's bike broke and this thing came out... From various sources I found that it's probably a cog from deraileur. He went to bike shops but none could find a replacement that's for this exact brand (and no one told him what brand it is even when he asked...). Therefore it would be great if anyone could give some pointers of what brand/size/etc so we could buy it off the internet (if you know where that would be really helpful too!)
I could also update with more info if needed, e.g. if you need the brand of the bike
It is a derailleur pulley, and fairly worn out one at that. The easiest way to find a fitting replacement is to buy a set from an aftermarket manufacturer that comes with adapter shims for different derailleurs. You can simply try different shims until you find the one that fits.

They do come in different teeth counts. From the picture it looks like this one used to have 11 teeth.