There are 3 ways often employed to keep a section of open water over ponds during cold winter: air circulation, water circulation and heaters.

There is quite a bit of information on this available on the web (e.g. google “de-icing koi ponds”)  Here is one example: http://www.drsfostersmith.com/pic/article.cfm?articleid=626

Also couple of Youtube videos:

https://www.youtube.com/watch?v=h9TryRiWE1A

https://www.youtube.com/watch?v=2Jrt10Dlc2A

Small floating pond de-icers are the least expensive ($40+), They only provide a small round window however, and some comments suggest they might not be sufficient in frigid temps.   The picture on Stoney Creek’s website provides an example.  http://www.stoneycreekequip.com/form/deice.htm
With techniques described in the second Youtube video this may be adequate. 

Water pumps and aerators designed specifically for fish ponds start at about $300 and go up.  

Pentair Aquatic Ecosystems has an aerator unit for small ponds for $350:
http://pentairaes.com/lake-management-equipment-and-supplies/lake-and-pond-aeration/air-pump-type/piston

Kasco Marine carries fairly heavy duty products
http://www.kascomarine.com/

All the above suppliers have reportedly good tech service people to help make a good selection for your need.