I have one that mentions baseball goods, I'll scan it in the next couple days.
A few other potential sources come to mind.
http://www.siegelauctions.com/
Has a search feature that covers all their auctions back into at least the 70's. I've never tried searching for baseball related covers, but it's worth a try.
the Frajola message board run by Richard Frajola is sort of like this place, just with a horrid bit of board software. But a lot of the guys that know "stuff" are on there. I've never joined formally, but read it fairly often. The site owner is a dealer in postal history - covers to the rest of us- and may be able to find covers for you. He may also be a *little bit pricy* (As in there's probably some really expensive stuff out there and a lot of that gets sold through connections like him)
http://www.philamercury.com/board.php
Steve B