Beginning in 1943, Hitler ordered that a small number of Sturmgeschütz III production be delivered to foreign countries. In most cases these were active allies of Germany (Spain being the exception). All of these allies signed separate armistices or changed sides by late 1944.
Bulgaria - Between February and December 1943 the Bulgarian armed forces received 55 Sturmgeschütz (most likely Ausf. G). These didn't see any action against the Soviets, but were rumored to have been used against the Wehrmacht when Bulgaria changed sides. The Bulgarians organized two Sturmgeschütz Abteilungen of 25 vehicles each.
The Bulgarian 1st Sturmgeschütz Abteilung was based in Sofia.
The Bulgarian 2nd Sturmgeschütz Abteilung in Plovdiv.
Finland- During the summer of 1943 the Finns received 30 Sturmgeschütz Ausf. G. A further 30 were shipped during Spring and early Summer of 1944. 15 were scheduled to be delivered in September of 1944, but were cancelled when Finland & Russia signed a cease fire on 9-Sept. In total the Finns fielded 59 StuG III's. Looking at the shipping inventory and the delivery numbers shows a discrepancy of one StuG. I have no evidence, but presume this was lost to accident or enemy action prior to the Finns taking delivery. Hungary - From August to September 1944 the Hungarians took delivery of 40 Sturmgeschütz Ausf. G. Italy - In May, 1943 the Italians took delivery of 12 Sturmgeschütz Ausf. G. After the Italian surrender the Germans "repossessed" the 12 Sturmgeschütz (along with 12 Panzer IV Ausf. G; 12 Panzer II Ausf. N; and 24 Flak 8,8cm guns). The Sturmgeschütz were reassigned to the 16th Panzer Division. Spain - In October, 1943 the Spanish took delivery of 10 Sturmgeschütz Ausf. G. Rumania - In the Winter of 1943 the Romanians took delivery of 6 Sturmgeschütz Ausf. G. From January to July of 1944 a further 94 units were delivered. Another 20 were scheduled in August, 1944 but due to the change in leadership (Coup d'Etat) the order was cancelled.
Sources GUGLIELMI, Daniele. Italian Armour in German Service 1943-1945. Mattioli 1885, 2005
SPIELBERGER, Walter J. Sturmgeschutz & Its Variants, (Spielberger German Armor & Military Vehicles Series, Vol 2). Schiffer Publishing, 1993