As we are a small developer, we don't handle instrument or accessory sales or replacements.
If you're still under warranty, you can contact Mad Catz or check your manual for warranty replacement procedures.
Mad Catz contact information is:
Web support: http://support.madcatz.com/
Email support: firstname.lastname@example.org
Phone Support: 1-619-683-2815 OR +44-(0)1454-451900
If you are no longer under warranty, or if your dongles were lost or stolen, the best place to find a replacement dongle is via an online retailer or reseller site, such as Amazon.com or eBay.com. Keep in mind that Harmonix is not affiliated with these sites and cannot guarantee any purchases made through them.