I would investigate GW's rather limited bits order service. They discontinued a lot of bits, but I don't think sonic weapons were on the list of things they stopped selling individually.
Otherwise, I'd try battlewagonbitz.com or thewarstore.com-- which are one and the same, really, but either place gets you to where you need to go. Unfortunately, due to GW's sort of annoying policies, you have to place orders for specific components with them over the phone-- GW decided no web orders for their products through third parties, unfortunately. I shop extensively at TheWarStore, and it's been a great resource for kitbashing things without spending the 35 USD to buy an entire box.