All sorts of places. Often it's an experience during your sexually formative years. Maybe you were attracted to someone making balloon animals at a carnival and now you have a latex fetish, maybe your first kiss was near a trash bin on a hot day and now you get turned on by the smell of rotting meat.

Can also be darker, a lot of people enjoy rough sex because of abuse as a young age.

It can also just be slightly different wiring in your brain. As I understand it the neurons that deal with your feet are in a similar place in the brain to those associated with erogenous zones so for many people, they crossover somewhat, hence the relatively common foot-fetish.