Celtic coinage from Bulgaria/Ancient Thrace (E-Book): https://balkancelts.wordpress.com/2017/05/10/c-c-c-h-b-celtic-coinage-and-coin-hoards-from-bulgaria/
Interesting subject. The map surprises me: apparently most hoards were found in inland Bulgaria, almost none at the coast of the Black Sea. By the way, I'm very cautious buying these coins because of the numerous forgeries from Bulgaria. It's much harder to recognize Celtic fakes than Greek or Roman coins.
Yes, most Celtic settlement appears to have been along the Danube and in the Thracian interior. Unfortunately, forgery and corruption are a way of life in Bulgaria...