To purchase locked items, you need to accumulate gold balls. You get credit for each gold ball you knock out of the park, a certain number of gold balls for winning matchups, and anywhere from twenty to thousands of gold balls for completing "achievements" in the game (win five matchups in a row, hit 50 home runs, etc.)
To see what it takes to earn each achievement, start the game, hit Play, tap the blue baseball player icon in the bottom left corner and then tap the green exclamation point icon in the top right. Tap on each achievement to see the requirements.
Lastly, once you've accumulated enough gold balls, tap on the icon for the equipment category you want to look at, then double tap whichever piece of equipment you want.