I would agree that Dropbox would be a good resource to use for being able to access your files and presentations easily. I see where you were looking for magnetic card readers, and while I don't personally know of any other than Square, I might be able to provide some other suggestions if I have more information. What specific goals do you have in mind at the trade show?
Are you looking for an app that is going to help you manage clients that you meet, if so I'd recommend SalesVerge, or just something to access your presentation materials and notes? May I ask also what kind of trade show you are visiting?