Options for replacing a broken kitchen faucet head?
For the individual faucet parts you will need to go to the manufacturer and purchase it from them (or another parts supplier... just search "kitchen faucet head" and you will find many different results).  You can also try just going to Lowes or Home Depot or your local hardware store and see if they have a generic replacement head.  The Home Depot site lists one: Spray-Head for Kitchen Pull-Out Faucets but it says it is only available in store.

But most likely the cost of the individual part (plus the shipping and handling) will be close to or more than putting in an entirely new faucet.  And since the head piece broke then most likely the rest of the faucet is getting old too so it might not be a bad idea to replace the entire kitchen faucet.  Replacing a kitchen faucet is not too hard; the hardest part is removing the old faucet (plus squeezing yourself under the sink).