A quite extensive agenda of organ concerts in The Netherlands can be found here: http://www.hetorgel.nl/nagconcert.htm. Unfortunately, in August no organ concerts in Zwolle are listed (yet). Looks like there are organ concerts once in two months only. But you'll find plenty of other interesting organ concerts
There are many sampled organs within 30-90 minutes driving from Zwolle.
Kampen 20 km
Raalte 25 km (sample set in development)
Vollenhove 35 km
Zutphen 45 km
Doesburg 65 km
Zeist 80 km
Utrecht 85 km
Anloo 90 km
Midwolda 120 km
Haarlem 135 km
Rotterdam 150 km
Many of those places are listed in the concert agenda which I linked above.