To my knowledge, there's no one answer to your question, but here are some menu items.
First, most cellular companies will sell you a USB dongle and a data plan. You may be able to use your modem with T-Mobile or AT&T. I don't know if anything like that is available in pre-paid cellular. That should work with your computer, but you might have to cable up the ipod.
Second, many if not all smart phones will have an access point functionality. You may have to pay double to use that capability -- Verizon, for instance, will charge you for a data plan for the smart phone, and then an extra charge for access point data.
Finally, many libraries, restaurants, and motels will have free WiFi. If you want to connect mostly in town, free is a great way to go. Broad swaths of rural America don't have cell access outside the towns, so you might not be limited much in where you can access the web.