Most solutions I have seen for a 1.8v uart are pretty expensive, especially if you include the import duty in many countries.
For example to get the official 96boards uart adaptor which costs $14, I need to pay $30 to get it shipped to India. And other solutions like the official usb to ttl module by ftdi still cost alot and other chips based on CPxxxx do not provide the reliability of an ftdi chipset.
So here is my solution using a ftdi based module rated for 5v and 3.3v that costs around $3-$4 and a 1117 1.8v LDO which I was able to get locally for just about $1 for a pack of 5, but is easily available for less than $1/pc
The procedure is simple,
1) connect the 5v pad on the module to pin 3 of the LDO
2) Connect pin 1 of the LDO to GND
3) Connect pi2 to the VCCIO or the Center pin on the Voltage select pins on the FTDI module
And you should be good to go, connect tx, rx and gnd to the 96boards CE.
For a bit more comprehensive explanation: