I bought mine at Cycle Therapy.. can't remember the brand. Mike?

It just attaches to the rails using little clips+straps so it's pretty easy to move. I just use it on my road bike though, so I don't really deal with that. I think mine is the smallest, fits what I need, but just barely.

-2* Park tire levers
-1 pack of Park glueless patches
-1 emergency first aid kit (another good thing to carry!)
-co2 + head
-extra tube
-multitool
-a couple ttc tokens

And then when I ride I throw a credit card and my ohip card in there, wrapped up in a $20 bill.