I know that they can be bought cheaply at sparkfun and others but when you only want to buy one converter you might pay more for the shipping than for the actual thing. And besides that, it is a simple thing to make.
You only need two bss138 mosfets, four 10k resistors, two sets of male headerpins and a pcb or a piece of veroboard.
Step 1: The Circuit.
Then make yourself a nice PCB-layout or find yourself a small piece of leftover veroboard and head to the next step.
Step 2: Soldering and Hook Up
I added a picture with the connections on it. The sda and scl could be swichted aslong as you switch them on the other side too.