For MLB logos you can check out Baseball Sim Central. The logos for OOTP 1-5 work for OOTP 6 as well. I'm a bit behind on updating the OOTP Download Center but when I'm done you can find plenty of logos there as well. If looking for any fictional logos the OOTP Logo Bank is a great place.
Baseball SimCentral
OOTP Logo Bank
The link for the Download Center is below in my signature. Hope that helps!