We aren't sure where it spead from exactly (most authors guess north-east gaul, usually without much discussion), but we can tell with moderate accuracy where it spread to assuming the rate of discovery of artefacts is approximately constant over the empire.

There are a lot of Epona artefacts in north-eastern Gaul, particularly in the Moselle valley and in the Burgundy region. There is a significant concentration on the Germano-Danubian limes, thinning out markedly in Germania Inferior and also thining suddenly in Raetia, to pick up slightly in Noricum and Pannonia.

There is a small cluster east of the Alps, off towards Acquincum and the danube bend, and another small cluster in Dacia. Other parts of the western roman empire have a small scatter - a few in Brittania, a couple in Hispania. One outlier in Macedonia (Thessaloniki), another outlier in Mauretania (Portus Magnus). None in Numidia, Egypt, Arabia, Syria, Asia, Galatia, Cappadocia - the 'Greek half' of the empire is practically empty.