There are several types of sensors you can use and at least 2 different mezzanine boards that can help with sensors.
First you can use any 1.8v sensor directly with no level shifter needed. There are many sold by DigiKey Electronics, Adafruit Industries, Unique & fun DIY electronics and kits, Seeed Studio and others. You do need to be careful to only buy 1.8v sensors as 3.3v and 5v sensors won't work directly attached to a 96Board.
The next choice is to use a mezzanine board with voltage level shifters on it. Right now the two that are prevalent are: 96Boards Sensor from Seeed Studios, and the Link Sprite board from Arrow Electronics - 96BOARDS-STARTER-KIT
Both of those boards have level shifters from 1.8V to 3.3V and 5V so you can use a wider range of sensors. Sensors can connect to the GPIO pins, to the I2C bus and to the SPI bus on the 96Boards or level shifted on the mezzanine boards.
Hope that helps.