Yes. Third-party teleconverters for Canon mounts do exist, including options from makers such as Kenko/Tamron/Soligor, and Sigma also makes teleconverters.

A key difference is compatibility. Canon’s own extenders have a protruding front element, so they only fit certain Canon telephoto lenses that are designed to accept them. Some third-party models, especially Kenko/Tamron/Soligor types mentioned by users, do not have that protrusion, so they can physically mount on a wider range of lenses, including some lenses that cannot take Canon extenders.

That broader fit can make them worth considering if you want more flexibility or lower cost. The tradeoff is that compatibility and performance can vary by lens and converter, so it’s important to verify fit and operation for your specific lens before buying.

So in short: yes, third-party options exist, and they can be useful—especially when you want a converter that fits more than Canon’s officially supported telephoto lenses.