Check with the house. I've seen everything from plastic to top of the line. Rule of thumb, the cheaper the league, the cheaper the ball.
Check the difference in league prices. Some of the ball leagues are a real rip off. You can take the difference and buy a better ball. Ran into that in one of our summer leagues.
League prices for the normal leagues were $8 a week. Ball league was $20. Came to $192 difference.
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~~~SrK - Have balls, will travel
Old bowlers never die, we just don't score as often