If it is an electronic mixer you are after then I just bought one on EBay for just under $5.00US. If you are interested the item number for the one I bought is 130646160182.
From that you will be able to search for a similar one. Good Luck!
Electrical tail/elevon mixers are as cheap as chips, in fact they are so cheap I have just taken one apart to make a y harness I think it was $4.99 and a y harness would cost me the same....they are cheap and they work