At the low cost end is something like the NetGear Mingle mobile hot spot and FreedomPop service. For $3.99/mo and a $30 purchase fee, I get 500MB (easily doubled for free to 1GB) per month of data. Higher data plans are available but pricing starts at $20/mo. It uses the Sprint 3G and 4G LTE networks - you can check coverage for the areas you expect to visit. 3G is manageable but slow for simple web access, and is what you will get outside of population centers. 4G LTE is fast, but likely only available near sizeable cities.
I use it (and the data plan on my smartphone) just for things like web surfing and audio stuff like podcasts or streaming radio while on the road. I have yet to go over 100MB per month. If you watch a lot of YouTube or other videos, you could need 10-100X more, and FP may not be the best provider for cost.
FreedomPop has an annoying habit of sneak billing for unexpected things whenever you make a service change, but I have found it simple and quick to correct.
There are similar hot spot options from probably all of the major mobile carriers and many of their resellers (like FP), but the few of those I've looked at have much higher equipment cost, and usually $40 monthly minimums. Of course, you get a lot more data for that. The FreedomPop deal works well for me, since I don't use it constantly, and you can even take it in and out of service whenever you want to suspend usage, for no charge. If you need 4G LTE service in the sticks, I don't know which carrier, if any, is best for that. No doubt it highly depends on where you are.
ETA: you're getting a lot of advice on different options, but they all boil down to about the same thing: you use either wifi (like at Starbucks) which someone else is paying for, or cellular/mobile data which you pay for. To access cellular data, you need either a mobile broadband card (like a USB stick or one that installs inside the laptop), or a hot spot, which is just a mobile broadband device that you access via wifi. With a hot spot, more than one user can access the service at the same time. It gets more complicated, because often you can turn your mobile broadband-enabled laptop or your smartphone with mobile data into a hot spot that others can access via wifi. The key is, you need a mobile broadband access device (stick, card, smartphone, hot spot), with a data plan that you pay for. There are basically three carriers that provide all the cellular infrastructure in the US - ATT, Verizon, and Sprint. Everyone who sells a plan, piggybacks off those networks. The quality of service (coverage and 3G vs 4G LTE) varies by carrier in different parts of the country, so you have to check into that as well. I don't believe you get roaming (ability to jump to someone else's network than the one you pay for, when they are the ones with service in your current location) with mobile data plans, but maybe in some cases you do for extra charge.
I'm no expert, but this is how I've come to understand this.
