Online source for baseboard molding
Check out jbros.com, site for Johnson Brothers. They have a catalogue of profiles you can view on line. Although they list the profiles with numbers with "jb" prefixes, they pretty much are the industry standard "wm" prefix profiles. Wherever you look, you will see profiles listed by a number such as "wm275" . The wm is the industry wide design specs for that profile as set by the Wood Moulding and Millwork Producers Association, so that in theory every manufacturers "wm275" is identical. I say "in theory", because even mouldings from different manufacturing runs of the same supplier will vary slightly, depending on the particular set of knives used for the run.
The big boxes usually have tear off sheets hanging on the moulding isles that you can take, and most lumber yards will have an in house catalogue of the standard wm profiles.
Take note that even if you find the profile at a big box, the thickness of the stock will most likely be thinner than the same profile from a lumber yard or mill supplier.
There are also lots of profiles in older buildings that were manufactured by local or regional millwork producers that had their own "in house" profiles but are no longer in business.