You can find the Elefant in CMFI and CMRT. In CMFI he is available from February till June 1944. In the formations tab you can find them under "Heavy Panzerjäger company (Elefant)" or you can purchase them individually.
On the eastern front (CMRT) the Elefant appears in October 1944 till the end of the war. However you can only purchase them within a formation, not individually. I guess that´s not intentionally and hopefully will be fixed sooner or later.
In both cases the Elefant came via additional modules, so probably make sure that you have those installed/activated.