Sketch comedy is a specialized skill. This means finding someone to teach that skill should be someone with a strong background in sketch. Finding teachers outside of Chicago, New York or LA is difficult. Rather than listing teachers here, which would constantly have to change, we provide some ideas for seeking teachers in your area.
Local theatres are an obvious choice. Sketches could be considered one-act plays and the same acting skills are useful in both forms.
This means that regular acting teachers would be able to teach these skills.
Comedy clubs can help you with lessons in comedy. Finding what works and what doesn't in terms of humor will prove very effective in making good sketch comedy.