I'd not heard of those and had to do a quick search. Found them for about $11 on Amazon.

Candace (Catty1) makes something similar, using a cloth "sleeve" for a regular cat collar, and spraying it herself with Feliway; as I recall she's had great success with that.

So glad the collars are working for you! I may try one for Mandy, she is just so panicky about any of the other cats entering "her" bedroom, and she never leaves it. Could help calm her, and doesn't seem it would hurt.

Hope they continue working! Any idea how long they are supposed to last?