One day admissions are available as a walk-up purchase the day you're there and there's no limit to how many they sell. As far as pre-purchasing those types of tickets online, I don't believe that's an option.
The free attendee figure is for Primus/Golden Ticket package holders only, but sometimes a limited quantity of them are sold online some weeks after the show - 2013's Machine Wars Starscream springs to mind. You can, with a day pass, buy any attendee souvenir sets that haven't sold out from the club store, but it's never easy to say how many will be around.
Finally, and this may be of a little bit of help to you, here's a mini "guide" to Botcon I made for last year's convention:
http://www.seibertron.com/transformers/ ... 014/30500/ some of those tips are geared towards staying for the week, but hopefully there's some good advice there as well as in the topic comments that followed it.
Have fun! Wish I could make it this year but life gets in the way sometimes.