Amazon.com has Sprint Sim Card Kit on sale for $0.99. Shipping is free with Prime or on orders $25+. Thanks LaughinGass
Note, be sure your phone is compatible with Sprint Network by checking here prior to buying Sim Card Kit.
Additionally, upon activation, you will receive $100 Pre-Paid MasterCard when you activate with Sprint when you register within 45 days of Activating Phone. Requires New Line and New Service and Activation Fee of $30 per line applies (more info).