What about asking a local woodworker to make you some .. it's only a pine plank and a router bit !

Trace the pattern/outline on a sheet of paper and contact your local cabinet makers/woodworkers in your area and see if they can find you the material or can fabricate it for you. You can even email or fax them the profile so they know exactly what to get you.

It looks like a 5 1/2" speed base. Click here. for the google page.

have you tried going to a real lumber yard? Or better, a shop that specializes in moulding and millwork? They may well have no online presence, but they do exist in most metropolitan areas.

That looks like a pretty common profile. Here's an example of the catalog from a company in the Boston area; http://www.andersonmcquaid.com/Anderson_McQuaid_moulding_profile_sheets.pdf

They have some similar looking baseboards. Similar companies will exist near you and can either give you what you need or make it for you.

Mill work shops have the ability, for a fee, to cut blades to match the profile that you want. They can then cut the board lengths that you need for your project. I suggest that you think ahead.... Are there any projects that you might complete in the future that might require this baseboard? How much will you need? Estimate, if possible. Then, have them mill enough to store for future use. I did this for a kitchen remodel and had enough milled for a bathroom remodel several years later. I stored it In my garage for eight years! Good luck!