Mylar balloons are great for parties!
Mylar balloons are the metallic variety available in almost unlimited designs. Everything from sunflowers to Winnie the Pooh to Big Bird and much more. You name it. There's likely a mylar balloon for it.
Some mylars come in special shapes. For example, Nemo and Marlin from the Disney / Pixar movie Finding Nemo were not the traditional round shape. I bet you can guess what shape they were...fish shaped.
Mylar balloons have several advantages over latex balloons. For one thing the designs are amazing in variety. For another, they stay inflated longer. A lot of longer. And they can be reinflated when they start to lose their float. Mylar balloons don't pop. The seam may blow out, but that takes serious overinflating to happen.
Drawbacks to mylars include their conductivity. If they are released outdoors, they can become entangled in power lines and wreak havoc.
To get single mylar balloons you'll likely want to find a local retailer. Unless you're tracking down a special design. Then eBay is a good option.