Candies similar to Shock Tarts (a.k.a. Shockers)?

As a child, Shock Tarts, now called Shockers, were one of my favorite candies. I remember I always used to get them at Target. I heard the candy was discontinued, but it was brought back under the name Shockers. Unfortunately, even those I cannot find. What is a sour, chewy candy like the Shockers?