Catnip Cigar, "El Gato Muy Loco"
http://woofandmeowshoppe.com/cacielgamuyl.html
has the best price I saw.
http://www.thecatconnection.com/page/TCC/PROD/BEST/CNP-0131
has a better picture.
This is the most potent catnip, and most popular toy, we've ever found for our cats. These cigars can be easily found online (prices vary, so look around), and in some cat shops, but not everywhere.
I highly recommend them. Store the cigars in a baggie in a place inaccessible to your cat (if there is such a place) and bring them out occasionally.
Disclaimer: I am in no way associated with the mfr of these cigars or any pet store... just cats.