Long telephoto lenses can be very expensive, but a reasonable alternative is a teleconverter, which attaches between your camera and lens and increases its focal length. However, teleconverters come with their own set of drawbacks. This excellent video discusses the use of teleconverters, including why you might want to consider one for your own work.
Coming to you from Will Goodlet, this great video discusses the use of teleconverters, including their pros and cons. Teleconverters generally come in two flavors: 1.4x and 2.0x, so named for the factor by which they increase the focal length of the lens to which they are attached. While they increase the reach of a lens for a relatively inexpensive amount, there are several general drawbacks you should be aware of. First, you lose either one stop or two stops (1.4x versus 2.0x, respectively) depending on which you use, and you may take hits to both your autofocus and image quality as well, though typically, the most pronounced effects happen with a 2x teleconverter, while you can usually expect good performance with a 1.4x device. Still, they can be a great compromise instead of investing in an entirely new lens. Check out the video above for the full rundown from Goodlet.