Feltan wrote:Well, since I used to be an officer with a map, I am not going to tell you.

Regards,
Feltan
REAL Officers don't need maps...
To answer the question, though, bring up the B Screen and mouse over what you want to buy. It will tell you where and how much is for sale, as well as how much is being bought.
Then, you can either take the shortcut and just left-click to buy in increments of 10, or go to the trade areas and use the T screen to buy however much you want.
I like to mix the two, depending on the circumstance.