You can simply buy all 30 stadiums by going into user the Community Market in Diamond Dynasty and buying the stadiums at the lowest available price. The prices range from 50-160. You'll need roughly $3,000-$4000 stubs in order to buy them all.
When you select a stadium to purchase, the details will show you if you already own one or not. If you do, obviously don't buy another one. You can also check the collection to see how many you currently have if you want to make a note of them before purchasing.
When you have all 30 purchased, go into your collection - stadiums - MLB - then check the stadiums and hit
to save changes.
If you want to rush this and get this early, note that you'll get 500 stubs just for submitting 15 stadiums. So you can buy until you run out of money, add 15 to your collection, then buy some more with that money.