I attend the games of my pro soccer team and have been allowed field access several times because of my BWL (Big White Lens). But pro Soccer here is like triple A baseball in other places, they are happy to have people show up period.
That said, try out your local semi-pro teams, college teams and high school. I don't live near triple AAA teams anymore, but when I did, I had no problem getting in with gear.
Make contacts and inquiries ahead of time. Join something so thou can say you are a part of something. The ____photo club... Heck, find a local newspaper and ask to be an unpaid intern/staffer.