There is also one for Advantix and for Advantage as well, pretty much the same deal... saw that a bit later after I was researching it. www.k9advantix.com go to special offers.. and I'm not sure what the website for the advantage is.
It's the manufacturer website... I don't think you can order direct from them.. I've found that pretty much anywhere else though, with the exception of a dollar or three... it's all about the same, unless you can order from the UK where it's a lot cheaper but the shipping time is really long.