Bobble is right...you can get logos done much, much easier than stadiums.
There are only a few stadiums designers and the best one, Teflon Skies, doesn't come on the boards anymore. It is very rare...maybe once every 6 months that we get a new stadium.
It is best to find the stadiums first, then pick your cities and nicknames to match the ballparks.
In addition to the above great site listed by Bobble, check out BigCity's and JoeRock's websites...they also have real and fictional stadiums to download.
JoeRock:
http://www.ootpdownloadcenter.com/
BigCity:
http://www.bigcitybaseball.com/home/index.php
(see Main Menu on left middle "BCB Downloads")