Not all Mints are created equally.
To refresh your memory, Mint is the name of a chain of baseball card stores scattered throughout Japan. I've seen some tucked away in shopping malls, and others with their own storefront. The Akihabara Mint is actually part of a toy store for adults (not be confused with an adult toy store).
The Mint store in Chiba is just a short walk from the main station by the same name, and is on an upper floor of a small mall.
The last thing I grabbed at Mint Chiba was the (at-the-time) newly released gymnastics box set. It features Japan's best male and female gymnasts and comes with an autographed or other special insert. I grabbed the set only, super-cheap since some people open the boxes for the specials only. In fact, I stopped in the Akihabara Mint last Sunday and saw a small stack of sets for 300 yen each (base set only, of course). Rie Tanaka is Japan's golden child in gymnatics, much like America has Kerri Strug, Carly Patterson, Shawn Johnson, and Nastia Liukin.
I was in a bit of a rush when I visited the Mint Chiba location, so it may have some hidden gems I'll discover on my next visit. I hope to find some Marines team issues that I know have to exist somewhere!
Mint Chiba is located on the third floor of the BEE-ONE building, next to Yodobashi Camera. Go out the west (south?) main exit, and follow the main street that terminates at the station (don't go under the tracks). It's basically the second building down the main street. Look for the (not-too-conspicuous but easily identifiable) Yellow Submarine logo. Here's a map to get a good visual of the area. You can visit daily from 9:30-8.