Normally, you could download it from
http://www.omnigroup.com/ftp/pub/software/MacOSX/10.5/ which contains Omni apps requiring 10.5 or later, such as OmniGraffle 5. However, some housecleaning appears to have taken place, and there aren't any older versions of OG there at this time. Best bet is probably to call up Omni when they turn on the phones in 2 hours (1-800-315-OMNI) and ask them to provide a copy of 5.2.1.