Another good way of getting an impression of the main street of various places (as they look now, obviously) is to use http://maps.google.com, zoom in on the downtown area, and then dragging the little man out to a street to get a street view.
Not all streets in all towns has a street view - only those streets in the satelite view that gets blue edges when you grab the little man has street view.
Had a look at Lockeford, CA. It does indeed have quite a few one story commercial buildings, but not in the style the original poster were looking for - wall to wall businesses - the busineses along the main street in Lockeford seems to be mainly free standing structures.
Oh, well - I guess it depends on what town the original poster wants to model what buildings would be appropriate for his main street.
I picked a couple of cities east of the Mississippi river at random and strolled down what appeared from the satelite photos to be their main business district.
E.g in Albany, Indiana, I found one one-story building that was attached to a taller neighboring building, and six-seven that were detached.
In Milton, Pennsylvania, I looked at South Front Road, which seems to be a business type area. Same there - not a lot of one story businesses wall to wall in a row with taller buildings.
In Monticello, New York, I looked at Broadway. Same thing - most row type commercial buildings seem to be two or three stories, didn’t spot off hand any one story businesses in a row.
Doesn’t prove a thing, of course - a sample of three from among tens of thousands (or more) small American towns is statistically insignificant.
Having a lot of one story buildings attached to two or three story buildings is certainly common in more urban areas - say in Hmongville along Rice Street in Mapplewood, MN (a suburb of Saint Paul).
Looks like this :
