Aleksandra, your question is a good one as there are numerous vendors that are tangential to DCIM but for whom DCIM is not core - like CFD vendors for example. With that, I’d recommend talking to the analysts about what is it you are looking for. Dave Cappuccio at Gartner has been looking into this market for quite a few years and has a firm grasp on its status. Also, Andy Lawrence with The 451 Group can share his insight as well as they have done extensive research around the major players. Forrester is another firm that can provide some insight into DCIM.
If you want some good reading (vs. just a listing of firms) Gartner recently released a report on 'How to Select and Implement DCIM Tools.' You can download it here (for a limited time so grab the report while you still can!):
In Focus on DCIM you'll find 11 software vendors.