Marellano did find the exact sticker, imo. I haven't found any stores or online sites selling them except Walgreen's. A search of product availability at Walgreen's in Orlando show that all 10 stores currently have them in stock. Apparently, it's a common sticker pack and readily available in the Orlando area.
Also, the pack comes in a variety of shapes and sizes. It seems that LE would have found a similar sticker or at least the remainder of the pack at the A home if KC had these.
On the other hand, some of the stickers included with this pack are similar to the cell phone type stickers that KC had on her phone. I've been looking for something in prior photos that look like the heart sticker found at the scene but I'll look for different sizes and colors now that I see how this sticker is sold.