Buy a prepaid SIM and tell them you want $25/1gb of data. They might force you to buy a $25 rate plan that goces you some minutes and texts. Dont turn on celluar data. Connect to wifi and go to APNChanger.com

Use the wiki to look up your GSM carrier's APN settings.

Using APNChanger's manual entry option, enter the correct settings and install the APN. You may now turn on cellular data. If it says you're not subscribed to a plan when you try to load a website, turn off data, uninstall the APN and repeat, paying careful attention to the APN settings - you typed them in wrong.

At the end of the month, set your data plan to auto renew and make sure you have enough money in your prepaid account so that it does. You'll be charged a few cents for texts and calls if you make them.

You also may have to cut the SIM down to fit in your iPhone.