Look at catalogs and advertising for clothing and consumer goods aimed for the middle class.
The Sears & Roebuck Catalog is wonderful resource for prices and styles for watches, clothing, appliances, tools, and even houses. A few early 20th century catalogs are available in the USModernist Library.
The Library has facsimiles of the 1897 and 1902 catalogs as well as several books with excerpts from the Sears catalogs. Unfortunately, there is no central online archive of these catalogs. Some of them are available on the web, including:
The sites above have old catalogs from other companies, such as Macy's and Abercrombie and Fitch.
Mail-order catalogs are still published today, such as the Neiman Marcus Christmas Book.
Sewing patterns are a wonderful way to find out what regular people wore during the 20th century. Catalogs (like Neiman Marcus above) and magazines (like Vogue) were often pitched towards a more upscale clientele.