I bought the lobster costume from the same maker last year. Edison did not look thrilled, but as with everything else in life, a few extra treats resigned him to his fate. ;)
Get the "medium" and not the "small". It's easy to forget that corgis are actually regular sized dogs, with very short legs.
http://www.nipandbones.com/ Check this site, if they don't have one ask and they may be able to get it for you. Tell them Princess Gwenie sent you. That is cute! Gwenie goes as Merdog again she love that dress. I did get her a new hat this week
Good luck!