The first thing you need to know, that prepaid SIM cards can be purchased in official stores or approved resellers. The credit itself you can buy at approved resellers as well as at supermarkets all over America.
Now, let's check the options in more details.
What are the options available for the SIM card with unlimited data in the USA?
Some plans may include a restricted amount of data, voice, and SMS, other USA sim-cards might include unlimited data, other ones sell pre-paid international sim-card which you can use both in the US and abroad. Some mobile providers may also suggest only unlimited data which is included in the pre-paid sim cards for usage in the USA. This data-only SIM plans without calls and SMS is extremely beneficial for modem and tablets users. Most of the telco operators provide 4G/LTE compatible sim-cards.
Prepaid SIM cards with unlimited data in the USA that comes with a smartphone.
Also, it is worth to notice, some prepaid sim card packs with unlimited data to use in the USA comes with the smartphone. What's that actually means. It is a prepaid USA SIM card with unlimited data in it, although it will be locked to one carrier. The trick here is that you cannot buy that sim card without buying a smartphone.
Prepaid international SIM card for using in the USA and abroad.
There is a different situation if you are a frequent traveler. Not only you need a better price and service locally, but also internationally. Regular roaming charges are vast and inconvenient in usage – you might even not be able to use your smartphone when abroad. The best option for you is still a prepaid SIM card, but with some extra features. For instance, a prepaid international sim card which is US-based might also be used in other countries and continents, if the card includes international roaming. The SIM chip will have data, voice, and SMS which can be unlimited both in the USA and abroad. It is quite rare, but there are some mobile providers by using which you can enjoy worldwide service and good quality.