You can find em for sale all over (both vintage or new)... or you can try your hand at making your own.
Here's a round-up of a few of our favorite DIY collar tutorials from around the web (two of em don't even require a sewing machine).
Angel Collar from Craftzine
No Sew Simple Felt Scalloped Detachable Peter Pan Collar by Honestly WTF
DIY Pearl Collar Necklace by Style Hive
Faux Fur Peter Pan Collar by Collette Patterns