I would like to use a radio for a plane requiring elevons but it does not have the correct mix. Is there a device that can be plugged between the reciever and servos to provide elevon mixing function?

CLBetten
Ace used to make a thing called a Cristi Mixer or similar that could do elevon or delta mixing. I am not sure if it's made any more or not.

DEG

Or you can do a manual mix like this:

Watt-Age (Hobby People) or Trick RC (the Zagi people) would be the place to look. They have elevon mixers for their foamie flying wings.

I like the Du-Bro "V-Tail Mixer" for mixing elevators and ailerons (elevons) on a flying wing or "tail-less" model, such as the Global Cutlass. As the name implies, you can also use it for such things as v-tail aircraft, like the Beechcraft Bonanza to mix rudder and elevator (ruddervators), or to mix flaps and ailerons (flaperons).

What I like about it is the components mount neatly to the top of your servo wheels - no "sliding tray" to fool with or anything to engineer - and because it uses ball links, there is no play or slop in the pushrod connectors or any part of the system.

If you don't have a computer radio (or a radio with a built-in mixer) this is a simple, effective way to go.

Check fmadirect.com they have an electronic mixer for elevons for $18.95.