Some Target stores do sell stamps, but not all Target stores. You will want to check with your specific Target store location and ask them if they sell postage stamps to be sure! Most likely Target Super stores (just like Walmart Supercenters), will have US postal stamps that can be purchased at the cashier or from the Customer Service desk.
It would be a good idea to call your Target store location to double check and make sure they sell stamps!
You can also purchase stamps online at Stamps.com
To find USPS stamps at store locations near you, just type in your zip code here: http://www.uspseverywhere.com/