I purchased a cricket SIM for $.99 when they had their $100 for switching a line credit promotion.
I ported one of my T-Mobile numbers, had a Cricket sim in two days and any att (locked or unlocked) device I've had works without issue, even unlocked T-Mobile devices have picked up LTE in my area.
So from this, you can get a sim and pop in -
The only thing, you have to give them the IMEI of the device you are going to use, they send the appropriate sized sim. In other words, you can't pick your sim size, unless I missed how to do that.
I had to order a second sim when I wanted to go micro to nano, but its easy through cricket chat.
You can even use chat to change sim numbers etc.
So far, I've been more than pleased with their customer service and wireless service. this will be my fourth month with them.