Ephebian is spoken in the Oasis, and in the Ephebian farmsteads. It can also be heard spoken by some of the crewmen which wander AM.
Ephebian may be taught to those of high spoken/written djelian skills in the Djelibeybi Language School, taught by Perdikas. The cost is 3 talents for the first lesson, 4 talents for the second, 6 talents for the third, and 8 talents for the last, per the standard of the school for all languages.
Morporkian speakers can get beginners' lessons in the Ephebian Embassy in Ankh-Morpork for &&&&&&&&&&&+2000 each. (Note that having learned some Ephebian here doesn't prevent you from taking higher-level lessons from the other school later on.)
As a warning for those with Cmud/Zmud, parsing should be turned off when attempting to repeat ephebian phrases in Djelian - the ; is a part of the sentence, and needs to be included for the statement to be correct. This can be done by clicking on the computer icon in the lower right corner, next to a gun icon - or, you can hit control+r.
The Chronides farmstead is a good spot to TM Ephebian. You can list the goods of the traders who hang around or just wander around a little listening to them. Female characters can also hang out in the women's quarters where (after 10:00 AM Disc time) Rhea and Koren Chronides chat with each other. If you have enough spoken Ephebian for Rhea to understand you, you can also ask her for a weaving lesson.
Outside where the guards are on Speedwell may also be a good spot.