Updated August 2015
I have found a few options for balloon shops in the area that have been recommended by others in the various Facebook groups. Consult their websites for opening times.
Balloon Shop Sittard
Steenweg 72, Sittard
Rodebachstr. 129, Gangelt
Drop Inn Geleen
Groenstraat 6a, Geleen
Winkelcentrum ‘De Parel’ 7, Brunssum
Marienstrasse 22, Übach-Palenberg
Toys R Us sells unfilled balloons and helium tanks.
Schumanstraße 4, Wurselen