Printrboard Rev. D. 3.3 V source
On the schematic, the 3.3v is marked as "U7" and pointed out here:



The 3.3v is marked as pin #2. You can find the three pints from U7 here on the board (blue circle):



I'm not exactly sure if the pins are accessed from the other side of the board, or even if they are marked at all on there. You can always check them with a multimeter to see what their output is. More than likely the bottom single pin is the ground and the top two are one or the other (5v &amp; 3.3v). 

As far as amperage draw, I'm thinking 45mA is not a lot of draw, but I'm not an expert. I'd think it should be able to handle it, but again, I really don't know.