The other major replacement shock companies are Progressive, Ohlins, Hagon, and Showa. Of the 4, there is almost no information on the Showa page and very little on the Ohlins page. Most of the information available will be from the Progressive and Hagon sites. Part number 32009 fits most, if not all, XS and XJ models. Options include black or chrome shock covers.
Price includes springs.
Differences are in type and number of external adjustments. The other models are less expensive. A 13 inch shock will allow you to use a lower preload, giving a ride that soaks up smaller bumps better. Some people have indicated they have adapted shocks from other bikes with good results. Since it was written in 1982, the solutions he advises are sadly out of date, but there is good information in the reading.