We can figure out how many gallons; what is the length, width, average depth, also is it round, oval, square, rectangle?
You can find help in your area! There are koi clubs all over the U.S. some states have a lot of them. My state only has one

. BUt, if you contact the koi club closest to you I am sure that they would be happy to help. Here is a link to the registered AKCA clubs.
http://www.akca.org/You may have more koi clubs or water garden societies in your area? If you do a web search you'll surely find them. There is always someone with extra stuff for emergencies, or just because it's there because they haven't gotten rid of it. Pumps, filters, water treatments, etc... I read an article about a koi club member whom was called because a family had a pond which had gotten out of hand. This pond had several koi and was disgusting. The koi club member went to the house, found out the problem and luckely had everything needed to fix the problems at home. It was a happy ending for everyone. People that love koi love all koi. They are more than happy to help other's when help is needed. This help is part of the reason that koi clubs were started. So get ahold of your local club and I bet you someone will be more than happy to help you.
Scott