A contingent with special offers for rooms in two different hotels can be found here: http://germany.nethotels.com/info/muenster/events/Geoinformatik/default_en.htm. These are available until 14th of May 2014.
To find further accommodation options in Münster or for bookings later than 14th of May please visit: http://www.muenster.de/stadt/tourismus/en/hotels.html.