What is the best option for mobile internet for a foreign traveller in the USA?

Next month I'm going to make a roadtrip through the USA for 21 days. Now, I'd like to have mobile internet (3G or 4G/LTE) during my whole trip. Preferably about 1 to 2 GB for not more than a total of $50.

I'll travel through CA, NV, AZ and FL (start in San Fransisco) and I prefer a sim-only prepaid card.

I can't find helpful information on the websites of AT&T, Verizon or T-Mobile. I only can find plans including a phone.

I've got an iPhone 5s GSM (A1457).

Your best bet is probably T-Mobile, if they have prepaid coverage in the area you're travelling (unfortunately they have one of the smaller nationwide networks). In the US, prepaid coverage is different than postpaid coverage, because most big carriers include roaming for free in their postpaid plans. Prepaid plans usually only get the 'native' network. If they get roaming it's usually talk/text only. This is T-Mobile's prepaid site, check the cities/routes you'll be travelling on their coverage map:

http://prepaid-phones.t-mobile.com/

Your phone is recent so I think it will be fine for the T-Mobile network. Here's more information on T-Mobile's prepaid offerings:

http://www.prepaidphonenews.com/2012/01/prepaid-carrier-profile-t-mobile.html

Unfortunately it's somewhat difficult for travellers to get SIM cards in the States because they aren't usually carried in the airport. T-Mobile may be the most accessible. They have stores in shopping areas, cities, and malls. You can also get some plans at Walmart. In fact, if you can get to a Walmart on arrival, I'd get the $30/month plan that you can only get at Walmart or online. It includes 100 minutes, unlimited texts, and 5GB of 4G data (unlimited throttled data). It also includes unlimited international texting. That would probably be your best bet, but it's only available at Walmart. Otherwise another T-Mobile prepaid plan will probably fit your needs if they have coverage everywhere you go.

Your phone is recent, so it's probably fine so long as it's unlocked. But double-check on that prepaidphonenews.com site.

T-mobile & AT&T are GSM, Verizon & Sprint are not.

T-mobile would be your best bet for sure, you can just get a t-mobile sim when you get here $10 usd, then buy minutes.

Assuming that you need prepaid and you need a cost effective solution you best bet would be T-Mobile, you need to check their prepaid coverage in your destination city. Prepaid plans usually have roaming charges but only for voice and sms.

Assuming that you are having your phone from your own country then you need to get a SIM card in USA which you can buy at some superstore say walmart where you can see multiple options as per your need.