In the US ask at Best Buy. I think Virgin has a pay as you go and there might be others.
Yes, Virgin Wireless does have such a plan available through Best Buy, Radio Shack, Target, and Walmart.
However, there are some significant features to keep in mind. First, you must purchase the mobile router ($149), not surprisingly. But you may be able to get a better deal through Verizon or AT&T if you already have cell phone service with one of those carriers.
Second, the "unlimited" plan is $50 per month and is "unthrottled" only to 2.5 gigabytes per month. That's sufficient if you do little streaming or use it only a few days per month. But if you're expecting to use it as a full-time internet connection, you may be disappointed. (My Verizon plan provides 5G per month at 3G or 4G speeds without throttling ($50) and completely unlimited access at $80 per month.)
Finally, you won't be able to access 4G speeds with the Virgin device. My Verizon mifi device downloads at from 5 to 15 times faster when I can use the Verizon 4G/LTE network. If you care about 4G access/performance do some research about the actual speeds available. "4G" networks are not created equal and some are several times faster than others.
All in all, if you're planning to use cellular-based internet a lot, you won't save much money (if any) with a pre-paid plan. If it's intermittent, say on an occasional vacation, it may be worthwhile. But in that case keep in mind the upfront cost to purchase the device.
P.S. Sprint also offers what they term 3G/4G mobile hotspot devices. The price of the device is about $50 (similar to Verizon) when you opt for a 2 year commitment. They have several plans which apparently don't enforce "throttling" (as I understand it.) However, the term "4G" is very loose. You may not experience the access speeds (or coverage) you can get from another carrier.
P.P.S. All of the above applies ONLY to the US. The options available in Europe, some of which are mentioned in this thread, are much better.