Could you convert dollar Bill's into nickels or do have to take your own nickels?

Actually you can bring your own nickels and/or convert your dollar bills to nickels with no charge

You buy the nickels at the counter.

Entrance fee is $2 per person

They have a machine to change bills to nickels or you can buy bags of nickels of either $5 or $10

5$ and 10$ bags of nickels are ready to go for purchase when you pay the entrance fee, and through out the visit. It's easy to exchange bills for nickels, and you can also bring your own nickels.