Hi folks! Hope your March and 2014 are going well!
I found this awesome Tumblr blog that features concept art from the Sonic cartoons. Not just promotional images and what-not, but actual character model sheets, too! Sonic Underground's in there - go ahead and take a look! pencilhill.tumblr.com/
For those who don't know how Tumblr works, you can either scroll down to view the images or in many cases, select the "archive" button located usually at the top or info area of the blog. This particular one doesn't have it, so if you just add "/archive" at the end of the address you'll get to it and view the images as thumbnails. Happy viewing!