Make sure that your prescription has your inter-pupillary distance written on it.
Most of the time, they just put the correction info, and not the spacing.
Without that number, you can't do it online.
If you don't have it, call and ask. If they don't have it written down, you need to get it.  It's pretty easy to do it if you have someone to help.
You need a mm ruler, and measure the distance between the pupils.  It's kinda like measuring for shot group size in that it's easier to use the inner edge of one, and the outer edge of the other instead if trying to guesstimate the center of the pupil.

An alternative technique is to put on a pair of glasses, and have someone use a dry erase, or soap marker to put a dot over each pupil, then you can measure on the glasses themselves.

http://static.zennioptical.com/media/Zenni-Optical-PD-Ruler.pdf