Problems with bluetoth on my Mac Mini

Hi guys! My bluetoth on my MacMini is not working anymore. I have a wireless keyboard and I paired it with my another machine with winndows and it is working. Where can I buy a USB Bluetoth adapter compatible with MacMini? Or what especifications I need to search to ensure it is compatible with my Mac.

Note: My Mac Mini is from 2011 I5 with 8 Gb of main memory and mouse and keyboard wireless (via bluetoth), my operational sytem is the last version, Yosemite.

Thanks, apple lovers!