While there are some aero posts, the degree of ellipticity you're measuring is more likely due to a post that has been distorted or an error inmeasurement. Errors in measurement can occur due to dirt in the caliper jaws, the caliper not being perpendiular to the post, foreign material on the post or a marred post. To avoid potential errors, it is best to measure well above the insertion point, where it is least likely to be marred or distorted. Also, clean the post and caliper jaws.

Based on the measurements it is probably a 25.4mm post. This is a very common size on entry level bicycles. You don't say what length the current post is, but posts up to 400mm long are readily available and quite inexpensive.