Hi,
you didn't give many details about your project (homogenity of background, light intensity, tracking speed, tracking accuracy, size of tracked light source, cost ...) - so answering now implies guessing for your needs.

And it's not exactly on-topic here, still: Have you considered using an extreme fish-eye lens on a USB camera? (They are very expensive but you can get 2 pi lenses).
Also you could try looking with an USB camera onto a metalized ball or cone and locate the reflex of your light source.
This is a "no moving parts" solution.

You can have a solution with movement around a single axis as well: use a clever combination of a linear sensor and one of those rotating light devices you'll find on some police cars (be careful when you pick one).

Greetings