There's not really such a thing as a "sponsor", but find someone in an Amish community who you can connect with, interact with, and who you can get to know. Tell them you are interested in joining their church and then they can put you in touch with Amish ministers and begin the process of becoming a member. If you don't know anyone in an Amish community you can get to know Amish people or an Amish family by visiting their businesses, buying produce, fresh eggs, visit any public-facing Amish business and talk to someone and they'll be able to introduce you to someone.