One of them is in the center, righ on the Independent Square. In the subway entrance there is an old church " Sveta Petka" . Near that church there is this shop.
information from another numismatic forum.
There is also:
1) an open "flea market" in front of the main church on the big square (and behind the Russian church) where you can find some sellers (weather dependent) - watch out for fakes from Bulgaria and Romania there though, they are frequently appearing as modern "silvers" as well as tempting fakes from ancient Rome!!
2) strangely there is a booth in the Central Market (facing the mosque) where they sell good quality old Bulgarian coins, plus recent commems (amongs vegetables and meat)
3) reportedly (I have never made it there) there is a coinshop at the Central Bank's building, where they sell their recent products
