craftgrrl | Help Finding a Dress to Modify // Building a Victorian Costume

This little task of mine has been driving me up the walls! I swear there should be claw marks.

I am working on building a Victorian costume for a monthly improv event for a character that is non-materialistic (by Victorian standards) so simpler is better. What I have been thinking to do is modify an old 70s white Gunne Sax dress to add a little bustle amongst other things.

The problem is that these don't seem to come in amazon sizes. I'm muscular, about 5'11, and will likely need to find a size 16 somewhere (normally I'd say 14 but Gunnies run small from what I hear), to alter. I know I'm not skilled enough to make an entire Victorian/pseudo-Victorian dress from scratch.

It feels like I've been searching everywhere. I've been on e-bay, resale shops, thift stores, internet businesses and NOTHING. Does anyone have any ideas of where I might be able to look to find one in an appropiate size? If not, does anyone have any ideas to help with this little costuming project?

Any help would be much appreciated. : /

Thanks!