In order to use Carista, you must have an ELM327-compatible Bluetooth device. This is a separate piece of hardware that you plug into your vehicle’s OBD2 port in order to allow Carista to communicate with your vehicle. Carista should work with all ELM327-compatible Bluetooth devices (v1.2 or newer for OBD2 diagnostics, v1.4 or newer for manufacturer-specific diagnostics and customizations), but some low-quality devices may perform poorly or fail to connect at all. We do not sell any hardware; however, many online retailers like Amazon do, and here are some example ELM327-compatible devices:
Disclaimer: due to differences between device manufacturers, we cannot guarantee that these will work. Please look at reviews for specific sellers, etc. and make your own decision.

BAFX Products (TM) - PIC18F2480 Bluetooth OBD2 scan tool - For check engine light and other diagnostics - Android compatible
DBPOWER Super Mini V1.5 OBD2 OBD-II Bluetooth CAN-BUS Auto Diagnostic Tool
LYL® Vgate ELM327 v1.5 Bluetooth Mini Small Interface OBD2 Scanner Adapter Torque Android