I bought the replacement snap ring from Bilstein Austria, so you can probably get some from Bilstein USA as well.
However, I agree with goldNSX - I don't think you'll need any. If you bend the snap ring open far enough to slide it into another groove, you only need to bend it open a few mm. And it's made of spring steel, so it's designed for that. I don't think it's designed to bend far enough for you to take it completely off the shock absorber at the shock absorber's thickest part. You'd need to bend it open several centimeters to do that and that's what I suspect my shop did. In any case, you shouldn't damage the snap ring in any way if you just carefully bend it open a few mm and then slide it into another groove.
When I put the snap rings into different grooves, I found I could see how the springs and the seats were oriented quite well just by sticking my head into the wheel well and using a flashlight. But there's usually more than one way to skin a cat. Have fun!