You can only buy them in sets of 2 or 4 (500 Atari Tokens for 2, 850 Atari Tokens for 4).
You need to log into the game, and click on the down arrow next to your mini-map (usually in the upper-right corner of the screen) and pick 'Visit the C-Store.' Then, pick the 'Services' tab on the left of the menu, and there's options for 2 and 4 Character slots.
Also, if you haven't gotten the 300-day Veteran reward yet, you'll receive a free character slot when you reach the 300 days. So, if you really only want 1 extra (and are willing to wait until you reach the 300 days), you could save some money that way.
Ah in game, I was looking at the web version of the C-Store. I'll check that out then. I'm not so waiting 300 days is do-able I suppose since I don't want to play all characters at once. Thanks for pointing me to the right location on how to find it.
I currently have the 2 slots from buying the game, the additional slot obtained at LT6, 2 slots from a Life Time Subscription and 4 slots purchased from the C-Store (brought during the sales).
I'll get another when i hit 300 days.
3 Fed's, 3 Klingons, 3 Romulans, with one spare slot left over. Thats the plan for the future. And since i love this game, in a few years when this has actually happened (this year i'm focusing one the 3 characters in my sig), maybe i'll get more slots for whatever is added later
And, if you don't need the slots just yet, i suggest waiting for another 20% sale, though since we have had 2 of these just recently, it could be a while.