Both Denver and St. Louis have Cultural Districts that are partially funded by regional sales taxes. These taxes are approved by voters for extended periods of time--I believe 8 or 10 years. By a negotiated formula, the tax proceeds are divided among a number of non-profits (museums, arts groups, botanical gardens and zoos). In both cities, the Zoo is the "leading" cultural group that generates the most support and receives the largest proportion of the funds, which are provided to help with basic operating costs (salaries, equipment and supplies). In the case of St. Louis, it allows (and requires) the zoo to continue its policy of free admission. The great thing is that this is money that is guaranteed during the life of the approval period, unlike most public funding which is subject to political variables and is a year-to year proposition. And sales taxes are far more stable than income or property taxes, so the zoos can budget with greater certainty.
In addition, Denver voters approved a very large bond issue (I believe $80 million) that is released dollar for dollar as private funds are raised for capital projects as outlined in the Zoo's master plan. This funded the new entrance and Predator Ridge, and will provide 50% of the upcoming Asian Forest exhibit, when the private match is achieved.
This approach is better than the year-by-year roll of the dice that many government-funded zoos have to live with, or to the "anything for a buck" necessity of many totally private zoos. And it is why Denver and St. Louis have such great zoos. I believe Columbus is supported by a similar long-term tax levy.