There are usually a lot of possibilities. If you are in a place that has a public library they will usually have internet connections where you can use the terminals(free of charge). Also there are places that will sell by the hour - places like Kinko's or independent internet cafes.

If you want to purchase by the hour and you have a laptop with a wireless card you can check out www.tmobile.com/hotspot and see where the hotspot locations are - a daily plan is $9.99 for 24 hours use. Many starbucks, borders books ,etc have hotspot locations. Other carriers also have similar plans such as at&t.

I think what you are referring to though is similar to a phone card for internet. you can check out this place as an example... http://www.maglobe.com/