Is there a single academic database for Management Information System /Decision Support System area?
The short answer is: No, there isn't.

This is simply because MIS/DSS research is often published in outlets which are more closer to the discipline of the context. For instance, many health informatics studies could also be found in the health outlets i.e., atypical MIS venues; or, a DSS study done in the context of nursing is most likely to be found in a nursing outlet, so on and so forth.