Idea # 2. Talk a friend, neighbor, or relative who otherwise might not do so in to getting their pet spayed. If necessary and within your means, offer to pay part of the cost or drive the animal to/from the vet for them if time or money is the issue. This link provides some talking points about the importance of spaying or neutering, and good responses to some of the common excuses people make for not doing so. Please use it to brush up before you broach the topic. Or even print the page and ask them to read it for themselves:
Idea # 3. If your area has a program to spay/neuter feral cats, you can cage-trap one of your neighborhood ferals and get him or her fixed. Your local shelter or rescue may be able to direct you to such a program, and the folks running it can provide further instructions. They may even be able to loan you a trap. Some programs charge a small fee to cover the cost of the surgery, but others are completely free. This site has a wealth of information on how to humanely trap cats:
Please share in the comments what you plan to do to cause the spaying or neutering of a pet that isn't yours, whether it's one of the options suggested above or your own idea, so that others can be inspired.