You might like to read what Doug has to say about medical kits.
http://www.equipped.org/medical.htm
Note his distinction between a medical kits for somebody in a more remote situation and first aid kits and materials for when help is close.
Doug has a list of what he thinks kits should have in them too.
http://www.equipped.org/survlkit.htm#MEDICALKITS

I have never bought anything over the internet. I have two stores I buy everything for first aid supply from in Edmonton and a couple of stores I use in other cities that I often am in.

For work site and corporate vehicle first aid kits I bought them already made up from industrial safety suppliers like North (Norton).
http://www.northsafety.com/
You need to click a few times to get to their lists of first aid kits and what they put in them. (These were bought because they comply with OHSA requirements and the compliance is documented then.)

I find all of these good things to look at and then I end up assembling my own private kits with what I know works for me.
I tend to be heavy on bandages and blankets, very light on drugs and lotions.