If all financing of government was voluntary, would anyone volunteer? The answer is: of course, if government were limited to its proper functions—the police, the courts, and the military. And you can see ample evidence of it everyday.
According to the Bureau of Labor Statistics, there were approximately 1,007,000 private security guards working in the United States in 2010. This is about 100,000 more than the number of police in 2008. According to the Department of Justice, Americans spend more than $100 billion per year on security alarms, security guards, and other security services, which is twice what is spent by federal, state, and local law enforcement departments combined. Clearly, Americans are voluntarily spending money in order to protect their property and persons. And this money is spent in addition to the taxes paid for the provision of police.
Many of the objections to the idea that government could exist without taxation arise largely because government has grown far beyond its proper purpose. Most taxpayers are rightly incensed when they hear of the many ways government wastes their money. Taxpayers understandably decry $600 toilet seats, programs that pay farmers to not grow crops, and welfare fraud. They correctly conclude that nobody would voluntarily support such wasteful spending.
With government limited to its proper purpose, all services except the police, the courts, and the military are provided by private companies. You are free to spend, invest, and donate your money as you deem best. If you believe that a business or non-profit organization is wasting your money, you are free to withdraw your support. In a capitalist society, if government officials insist on wasting your money, you are also free to withhold your financial support. Try doing that today.