Christine:
The TriStar website for the United States is www.tristarclean.com . To access the model history, click the "About Us" button on the home page, then click on "History of Products" when it appears in a drop-down menu.
The TriStar Canada website is www.tristarvacuumscanada.com . The vac models are shown on the front page, but there isn't as much info as the U.S. site has (and the U.S. site doesn't have all that much).
If you want info, the U.S. site is better. If you want to order parts, I really recommend the TriStar Canada website over the U.S. one. TriStar Canada stocks parts for direct ordering. If you call TriStar in the U.S., they refer you over to a local authorised repair facility. My experience has been that the local facility has to order parts from corporate, and then ship them to you, which is very time-consuming. The Canadian system is way more efficient and faster. I got my parts order from them in about one week.
The Canadian site also has parts schematics in PDF format, which can be downloaded and printed for future reference. The schematics are only available for the EX-20 (same as the CXL and DXL models), and the current generation of TriStars. But they're very good, with every part and every part number shown, as well as a wiring diagramme.
Hope this helps.