Nobody can tell you which device to buy.
You will need to research them. Search Google.
You will need to find a device that meets your requirements, and look at things like;
1) connectivity to the host, usb, serial, ethernet
2) Type of sensor, optical / capacitance etc.
3) availability of SDK's
Search for articles on Biometrics to gain an understanding. e.g.
A framework in C# for fingerprint verification[
^]