Amazon is also an option, but don't get too excited with too cheap unknown brand China products! They don't last long. Well, actually it depends what You choose for what and how You ride
But, anyway, if You have a good chain it's better to go for good sprockets. Everything will last much longer. Stock sprockets and chain with proper care can last more than 15-20k miles.
I don't say that Chine kung fu always no good.
Different metals have different wear levels. If sprockets are too hard they will eat Your chain off faster. Cheap hard ones can even break. If they are too soft, they don't last long and can even bend. There are also aluminum and titan sprockets for specific racing, stunts, whatever. They will not be appropriate for everyday use.
In my opinion, good aftermarket rear ordinary sprocket for ordinary oring chain should cost not less than 40 (or even 50) bucks. In way of hardness it should be compatible with Your (ordinary, if You have such one) chain.
And if there's time to change whole set, the best way is to buy a chain with sprockets in one kit.
So, be careful!
Bagger_Vance, sorry for my philosophy
I'm not trying to teach or judge anyone here. I just hope it helps someone who is trying to figure out same sprocket thing
In most cases ordinary steel sprockets should be ok.