Make certain ATC knows where you are! Use a Transponder and Encoder for Mode "C" operation.
There are two types of Encoders: Encoding Altimeters and Blind Encoders. A Blind Encoder is much more cost-effective, so unless you need to replace an existing Encoding Altimeter, we recommend a Blind Encoder.
A Blind Encoder is basically a remote altimeter, with the pressure always set to 29.92". You don't see its reading, but it is transmitted to ATC through the transponder.
Even though we are in the used avionics business, we find that a used Blind Encoder rarely makes economic sense. By the time we repair and certify a used unit, it's usually within a few dollars of a new one. As a result, we usually supply new Blind Encoders.
Our customers have had good success with Trans-Cal, but if you have a preference, we can supply Blind Encoders from ACK, Ameri-King, etc.
Regardless of the Blind Encoder you choose, you will need a Transponder to use with it.